SDI: Technology, Survivability, and Software. May NTIS order #PB

Transcription

1 SDI: Technology, Survivability, and Software May 1988 NTIS order #PB

2 Recommended Citation: U.S. Congress, Office of Technology Assessment, SDI: Technology, Survivability, and Software, OTA-ISC-353 (Washington, DC: U.S. Government Printing Office, May 1988). Library of Congress Catalog Card Number For sale by the Superintendent of Documents U.S. Government Printing Office, Washington, DC (order form can be found in the back of this report)

3 Foreword In its 1985 report, New Ballistic Missile Defense Technologies, OTA attempted to place those technologies against a useful policy background for the Congress. While that report introduced the major subject areas of Strategic Defense Initiative research, the amount of detailed technical evaluation it could offer was limited. The chief limitations were the relative newness of the SDI program and the lack of specific BMD system architectures to examine. Since that report, the SDIO has conducted enough additional research and, in particular, identified a sufficiently specific system architecture that a more detailed OTA review of the relevant technologies should be helpful to Congress. Public Law (continuing appropriations for fiscal year 1986) called for the Office of Technology Assessment to conduct a... comprehensive classified study... together with an unclassified version... to determine the technological feasibility and implications, and the ability to survive and function despite a preemptive attack by an aggressor possessing comparable technology, of the Strategic Defense Initiative Program. In addition, the accompanying Conference Report specified that... This study shall include an analysis of the feasibility of meeting SDI computer software requirements. This unclassified report completes OTA s response to that mandate. It puts SDI technologies in context by reporting the kinds of ballistic missile defense (BMD) system architectures that the SDI organization has considered for phased deployment. It reviews the status of the various SDI technologies and system components. It analyzes the feasibility of producing dependable software of the complexity that advanced BMD systems would require. Finally, it summarizes what is now known and unknown about the probable survivability of such systems against concerted enemy attacks of various kinds. The study found that major uncertainties remain concerning the probable cost, effectiveness, and survivability of the kinds of BMD system (which rely on kinetic rather than directed-energy weapons) that might be deployable in the phase-one proposed for the mid to late 1990s. In addition, OTA believes several more years of SDI research would be needed to determine whether it is feasible to construct the kinds of directed-energy weapons contemplated as follow-ons to SDIO s phase one BMD system. The survivability of both short-term and longer-term BMD systems would depend heavily on the outcome of a continuing competition in weapons and countermeasures between the United States and the Soviet Union. Finally, developing dependable software for advanced BMD will be a formidable challenge because of the difficulty of testing that software realistically. OTA gratefully thanks the hundreds of individuals whose contributions of time and effort helped make this report possible. OTA, of course, bears the final responsibility for the contents of the report. Director Ill

4 Advisory Panel on SDI: Technology, Survivability, and Software Robert Clem Director of Systems Sciences Sandia National Laboratories Malcolm Currie Executive Vice President Hughes Aircraft Company H. Guyford Stever, Chairman Foreign Secretary, National Academy of Engineering Gerald P. Dinneen Corporate Vice President for Science & Technology Honeywell, Inc. Peter Franken Professor Optical Sciences Center University of Arizona John Gardner Vice President for Engineering & Operations McDonnell Douglas Astronautics Co. Richard L. Garwin IBM fellow IBM T. J. Watson Research Center O Dean Judd 1 Chief Scientist for Defense Research & Applications Los Alamos National Laboratory Michael M. May Associate Director at Large Lawrence Livermore National Laboratory Stephen Meyer Associate Professor Center for International Studies Massachusetts Institute of Technology David Parnas Department of Computing and Information Science Queens University Charles Seitz Professor Computer Sciences California Institute of Technology John Shore Director Washington Research Lab Entropic Processing, Inc. Jeremiah D. Sullivan Professor Department of Physics University of Illinois Samuel Tennant Vice President The Aerospace Corporation Victor Vyssotsky Director Cambridge Research Laboratory Digital Equipment Corporation Gerold Yonas Vice President The Titan Corporation Charles A. Zraket President The MITRE Corporation Invited Observer: Chief Scientist Strategic Defense Initiative Organization Currently, Chief Scientist, Strategic Defense Initiative Organization. NOTE: OTA appreciates and is grateful for the valuable assistance and thoughtful critiques provided by the Advisory Panel members. The views expressed in this OTA report, however, are the sole responsibility of the Office of Technology Assessment. Participation on the Advisory Panel does not imply endorsement of the report. iv

5 OTA Project Staff SDI: Technology, Survivability, and Software Lionel S. Johns, Assistant Director, OTA Energy, Materials, and International Security Division Peter Sharfman, International Security and Commerce Program Manager Thomas H. Karas, Project Director Anthony Fainberg C. E. Sandy Thomas David Weiss Administrative Staff Jannie Coles Cecile Parker Jackie Robinson Acknowledgments The following organizations generously made their personnel available to provide OTA with information and ideas Aerospace Corporation American Physical Society, AT&T Bell Laboratories Avco Systems/Textron Boeing Booz-Allen & Hamilton Ford Aerospace and Communications General Research Corporation Hughes Aircraft Company Lawrence Livermore National Laboratory Lockheed Missiles & Space Company Los Alamos National Laboratory LTV Martin-Marietta McDonnell-Douglas M.I.T. Lincoln Laboratory MITRE Corporation Nichols Research Rockwell International SAIC Sandia National Laboratory Sparta Strategic Defense Initiative Organization System Planning Corporation TRW University of Texas, Center for Electromechanical Engineering U.S. Air Force Electronic Systems Division U.S. Air Force Space Division U.S. Air Force Weapons Laboratory U.S. Army Strategic Defense Command U.S. Naval Research Laboratory U.S. Naval Sea Systems Command W. J. Schafer & Associates Westinghouse Marine Division

6 Workshop on Soviet Response to SDI, January 1987 Sidney Graybeal, Chairman Vice President, System Planning Corporation Alex Gliksman, Convener/Rapporteur Consultant Arthur Alexander The RAND Corporation Mark M. Lowenthal Director, INR/SFA US Department of State Arthur F. Manfredi, Jr. Assistant National Intelligence Officer Central Intelligence Agency John A. Martens ITA/Office of Foreign Availability U.S. Department of Commerce Stephen Meyer Professor Center for International Studies Massachusetts Institute of Technology Robert Nurick Co-Director RAND-UCLA, Soviet Studies Center The RAND Corporation Sayre Stevens President, System Planning Corporation Keith Taggart 1 Assistant Director/Countermeasures Strategic Defense Initiative Organization U.S. Department of Defense Robert D. Turnacliff Principal Director, Threat Analysis Office The Aerospace Corporation Vann H. Van Diepen INR/SFA U.S. Department of State Currently with SAIC, Inc. Workshop on SDI Software, January Larry Druffel, Chairman Director, Software Engineering Institute Carnegie-Mellon University William Ainsley Logicon, Inc. Speakers Mack Alford Senior Software Scientist General Electric Corporation Karl Dahlke Technical Staff AT&T Bell Laboratories Discussants Bruce Arden Dean of Engineering & Applied Science University of Rochester Richard Kemmerer Department of Computer Science University of California, Santa Barbara 1987 Butler Lampson Corporate Consulting Engineer Digital Equipment Corporation Brian Reid Consulting Engineer Digital Equipment Corporation Panelists David Parnas Professor Department of Computing and Information Science Queens University John Shore Director, Washington Research Lab Entropic Processing, Inc. Victor Vyssotsky Director, Cambridge Research Laboratory Digital Equipment Corporation vi

7 Contents Chapter Page 1. Summary Introduction Designing a Ballistic Missile Defense (BMD) System: Architecture and Trade-Off Studies Status and Prospects of Ballistic Missile Defense Technologies, Part I: Sensors Status and Prospects of Ballistic Missile Defense Technologies, Part II: Weapons, Power, Communication, and Space Transportation System Development, Deployment and Suport System Integration and Battle Management Computing Technology Software Nondestructive Countermeasures Against Ballistic Missile Defense Defense Suppression and System Survivability Defense Suppression Initiative Appendix A: Technology for Producing Dependable Software Appendix B: Glossary vii

8 Preface This report is the unclassified version of a classified document delivered to Congress at the end of August In attempting to reach agreement with the Department of Defense on what information could be included in an unclassified report, OTA found the wheels of bureaucracy to turn very slowly when they turned at all. Only through the active intervention of the Strategic Defense Initiative Organization, beginning in late in November 1987, and extending to the end of March, 1988, was a partial resolution of the problem achieved. OTA, with assistance from SDIO staff, revised the entire report to produce a complete version that both agreed should not be considered classified. The Department of Defense concurred on all but the final three chapters. These latter chapters deal-in a general way and without the kind of specific detail that might be useful to an adversary-with a variety of potential countermeasures to BMD systems. In particular, chapters 11 and 12 deal with defining and countering threats to the survivability of space-based BMD systems. Chapter 1 offers a brief review of the bottom lines of chapters 10 through 12. But apparently some in the Defense Department wish to assert that it is impossible to present an unclassified analytical discussion that would enable the reader to understand the issues and form his own judgments. In OTA s judgment, this position does not deprive potential adversaries of any information they do not already have: rather, it stifles rational public debate in the United States over the pros and cons of proceeding with ballistic missile defense. To give the reader at least some appreciation of the scope of the deleted material, the tables of contents of chapters 10 through 12 appear at the end of this volume. In addition, the major conclusions of these chapters (without, of course, the supporting analysis) are summarized in chapter 1. OTA thanks the SDIO for the additional substantive comments and information it provided on the final drafts of the report. Thus, despite the many months of delay since original completion of the report, this unclassified version is reasonably up to date. OTA, not SD IO, is responsible for the contents and conclusions of the report. A further note on the subject of classified information is in order. Any report which attempts to analyze the feasibility and survivability of prospective ballistic missile defense systems must refer to possible measures an adversary could take to counter the system. OTA sought the views of a variety of experts on Soviet military research, development, and deployment about potential responses to the SDI. It also sought to understand the technical feasibility of various countermeasures. It did not seek out or report on the official judgments of the U.S. intelligence community on what countermeasures the Soviet Union would or could take against SDI-derived systems. Therefore, nothing said in this report should be construed as an intelligence judgment of Soviet intentions or capabilities. viii

9 Chapter 1 Summary

10 CONTENTS Principal Findings Introduction Origin of This Study Goals of the SDI Nature of This Report First-Phase Technologies and Systems Goals of First-Phase System Technical Feasibility of Sensors and Weapons Software Feasibility Survivability of a First-Phase System... Second-Phase Technologies and Systems... Goals Technical Feasibility Software Feasibility Phase-Two Survivability Third-Phase Technologies and Systems.... Goals Technical Feasibility Software Feasibility Phase-Three Survivability Important General Issues costs Timing and Evolution Competition in anti-satellite Weapons , ,..., , Box Box l-a. Adaptive Preferential Defense Figures..... Page Figure No. Page l-1. The Path to Thoroughly Reliable Defenses Development Decision Content Mission Effectiveness Improves With Phased Deployment OTA Understanding of Pojected Roles of BMD Deployment Phases SDIO Proposal for Development and Deployment Table No. Page l-1. SDIO s Phase One Space- and Ground-Based BMD Architecture l-2. OTA s Projections of Evolution of Ground-and Space-Based BMD Architecture

11 Chapter 1 Summary PRINCIPAL FINDINGS The Strategic Defense Initiative Organization (SDIO) currently advocates planning for a three-part phased deployment of ballistic missile defense (BMD) systems, with each phase providing an increment of strategic benefits while preparing the way for the next phase. The first phase would be intended to... compel Soviet operational adjustments and compromises by reducing the confidence of Soviet planners in predicting the outcome of a ballistic missile attack. The second phase would be intended to negate Soviet abilities to destroy many strategic targets, and the third to eliminate the threat posed by nuclear ballistic missiles. The exact composition and timing of each phase are still under study, but some tentative system architectures have undergone preliminary analysis. Finding 1: After 30 years of BMD research, including the first few years of the Strategic Defense Initiative (SDI), defense scientists and engineers have produced impressive technical achievements, but questions remain about the feasibility of meeting the goals of the SDI. The SDIO has identified most of the gaps between today s technology and that needed for highly effective ballistic missile defenses; it has initiated programs to address those gaps. It should surprise no one that many technical issues remain unresolved, especially when one considers that the SD I has so far had time and authorization to spend only a fraction of the money that the Fletcher Commission estimated would be necessary to assess BMD feasibility. The SDIO argues that application of sufficient resources will resolve the outstanding issues. Finding 2: Given optimistic assumptions (e.g., extraordinarily fast rates of research, development, and production), the kind of first- Note: Complete definitions of acronyms and initialisms are listed in Appendix B of this report. phase system that SDIO is considering might be technically deployable in the period. Such a system might include: space-based hit-to-kill vehicles for attacking missile boosters and post-boost vehicles (PBVs) and ground-based rockets for attacking warheads before reentry into the atmosphere. Depending on whether U.S. deployment schedules could be met, the effectiveness of countermeasures that should be available to the Soviets in that period, the numbers of offensive weapons they had deployed, and the nature of the attack, such a system might destroy anywhere from a few up to a modest fraction of attacking Soviet intercontinental ballistic missile (ICBM) warheads. Again depending on the effectiveness of Soviet countermeasures, the BMD system might be able to carry out a strategy of adaptive preferential defense, allowing it to protect successfully a useful fraction of certain sets of U.S. military targets. 1 Additional defense capabilities would soon be needed to sustain this level of defense against either increased or more advanced, but clearly feasible, Soviet offenses. One key to sustaining and improving defense capabilities in the period would be development of technologies to discriminate between missile warheads and decoys so that ground- and satellite-based rockets could effectively attack warheads in space. Assuring functional survivability of space-based systems would also be essential (see Finding 4). 1 SDIO officials argue that denial to the Soviets of high confidence of destroying as many of these targets they would like (as estimated by U.S. planners) would enhance deterrence of an aggressive nuclear attack. 3

12 4 As the Soviets phased in faster burning, faster weapon-dispensing ballistic missiles, it would probably be necessary to develop and deploy directed-energy weapons to intercept missiles in the boost phase and post-boost phases. Given higher annual funding levels than so far appropriated, the SDI research and technology program might establish in the midto-late 1990s whether the components needed for warhead/decoy discrimination in a secondphase system would be feasible for deployment in the period. Also assuming higher funding levels than in the past, by the mid-tolate 1990s the SDI may determine the technical feasibility of deploying BMD directedenergy weapons in the period. The cost and survivability of such weapons will be among the key issues. Finding 3: A rational commitment to a phaseone development and deployment of BMD before the second and third phases had been proven feasible, affordable, and survivable would imply: a) belief that the outstanding technical issues will be favorably resolved later; b) willingness to settle for interim BMD capabilities that would decline as Soviet offenses improved; or, c) belief that U.S. efforts will persuade the Soviets to join in reducing offensive forces and moving toward a defensedominated world. Finding 4: The precise degree of BMD system survivability is hard to anticipate, because it would depend on the details of measures for offensive attack on the BMD system and defensive countermeasures, on the tactics employed by each side, and on the inevitable uncertainties of battle. It appears that direct-ascent nuclear anti-satellite weapons (DANASATs) would pose a significant threat to all three defense system phases, but particularly to the first two. Numerous DANASATs could be available to the Soviets in the mid-1990s (e.g., ballistic missiles relying on mature technology, could probably be adapted to this role.) Such weapons deployed in quantity, especially with multiple decoys, would threaten to degrade severely the performance of a first- or secondphase BMD system. SDIO officials say, however, that adequate survivability measures could meet this threat. If the Soviets chose to attack the U.S. BMD satellites during emplacement, they might prevent full system deployment and operation altogether. Finding 5: There has been little analysis of any kind of space-based threats to BMD system survivability. SDIO analyses assume that U.S. BMD technologies will remain superior to Soviet technologies (although such superiority would not necessarily guarantee U.S. BMD system survivability). In particular, SDIO and its contractors have conducted no serious study of the situation in which the United States and the Soviet Union both occupy space with comparable BMD systems. Such a situation could place a high premium on striking first at the other side s defenses. The technical (as well as political) feasibility of an arms control agreement to avoid such mutual vulnerability remains uncertain. Finding 6: The survivability of BMD systems now under consideration implies unilateral U.S. control of certain sectors of space. Such control would be necessary to enforce keep-out zones against Soviet anti-satellite weapons or space mines during and after U.S. BMD deployment. Most BMD weapon technologies would be useful in an anti-satellite role before they reached the levels of power and precision needed for BMD. Thus, the Soviets would not need to achieve BMD capabilities to begin to challenge U.S. control of, or even access to, space. Finding 7: The nature of software and experience with large, complex software systems indicate that there may always be irresolvable questions about how dependable BMD software would be and about the confidence the United States could place in dependability estimates. Existing large software systems, such as the long-distance telephone system, have become highly dependable only after extensive operational use and modification. In OTA's judgment, there would be a significant probability (i.e., one large enough to take seriously) that the first (and presumably only) time the BMD system were used in a real war, it would

13 5 suffer a catastrophic failure. 1 The complexity of BMD software, the changing nature of system requirements, and the novelty of the technology to be controlled raise the possibility that the system may not even be able to pass the more realistic of the peacetime tests that could be devised for it. The relatively slow rate of improvement in software engineering technology makes it appear unlikely to OTA that this situation will be substantially alleviated in the foreseeable future. SDIO officials assert, however, that SDI software problems will be manageable, that adequate testing will be possible, and that previous military systems have been deployed without complete system testing (e.g., the Minuteman missile system, the Navy s AEGIS ship defense system.) Finding 8: No adequate models for the development, production, test, and maintenance of software for full-scale BMD systems exist. Systems such as long-distance telephone networks, early missile defense systems such as SAFEGUARD, the AEGIS ship defense system, and air traffic control all differ significantly from full-scale BMD. The only kind of BMD system for which the United States has software development experi- 1 In ch. 9 catastrophic failure is arbitrarily defined as a decline of 90 percent or more in system performance, and there is a discussion of alternative approaches to the concept. ence is a terminal defense system. Incorporating a boost-phase defense would add complexity to the software and require the inclusion of technologies hitherto untried in battle. Adding a mid-course defense would probably increase the software complexity beyond that of any existing systems. Experts agree that new methods for producing and safely testing the system would be needed. Evolution would be key to system development, requiring new methods of controlling and disseminating software changes and assuring that each change would not increase the potential for catastrophic failure. OTA has found little evidence of significant progress in these areas. Finding 9: There is broad agreement in the technical community that significant parts of the research being carried out under the SDI are in the national interest. There is disagreement about whether or not this research is best carried out within a program that is strongly oriented toward supporting an early 1990s BMD deployment decision, and that includes system development as well as research elements. This question was outside the scope of OTA s mandate and is not addressed in this report. INTRODUCTION Origin of This Study The appropriations continuing resolution for fiscal year 1986 (Public Law ) called for the Office of Technology Assessment to produce a comprehensive classified study... together with an unclassified version... to determine the technological feasibility and implications, and the ability to survive and function despite a preemptive attack by an aggressor possessing comparable technology, of the Strategic Defense Initiative Program. In addition, the conference report accompanying this legislation specified that this study shall include an analysis of the feasibility of meeting SDI computer software requirements. This report responds to that legislation. After 30 years of BMD research, including the first few years of the Strategic Defense Initiative, the dedication and ingenuity of thousands of U.S. scientists and engineers have produced many impressive technical achievements. Such achievements may someday cumulate to form the basis for a highly effective BMD system. For now, however, many questions remain about the feasibility of meeting SDI goals. Goals of the SDI According to SDIO s annual report to Congress: From the very beginning, the SDIO has maintained the same goal-to conduct a vig-

14 6 orous research and technology development program that could help to eliminate the threat of ballistic missiles and provide increased U.S. and allied security. Within this goal, the SDIO's task is to demonstrate SDI technology and to provide the widest range of defense options possible to support a decision on whether to develop and deploy strategic defenses. Such defenses might, to a greater or lesser degree, protect the American population from nuclear weapons. But, contrary to the perceptions of many, SDIO has never embraced the goal of developing a leakproof shield against an unconstrained Soviet nuclear weapon threat. It is the position of SDIO that President Reagan has not embraced that goal either. 3 Rather, the organization, in its first 4 years, worked out a scenario that it argues could lead to President Reagan s stated ultimate goal of eliminating the threat posed by strategic nuclear missiles... [which could]... pave the way for arms control measures to eliminate the weapons themselves. 4 The scenario, paraphrased from the SDIO report, is as follows: a research and development program continues until the early 1990s, when a decision could be made by a future President and Congress on whether to enter into fullscale BMD engineering development; the Defense Department begins full-scale development of a first-phase system while continuing advanced technology work; the United States begins phased deployment of defensive systems, designed so that each added increment of defense would enhance deterrence and reduce the risk of nuclear war ; although this transition period would preferably be jointly managed by the United States and the Soviet Union, U.S. deployments would proceed anyway; then 4. the United States completes deployment of highly effective, multilayered defensive systems, which could enhance significantly the prospects for negotiated reductions, or even the elimination, of offensive ballistic missiles. Figures 1-1 and 1-2 are SDIO graphic representations of its development and deployment policies. Figure 1-1 illustrates that, as time goes on, newer, more capable BMD systems would be necessary to respond to advanced Soviet missile threats. Alternatively, it is argued, the prospect of such new systems might persuade the Soviets to accept U.S. proposals for joint reductions of offensive forces which might, in turn, obviate the need for new systems. Figure 1-2 lists the kinds of information SDIO seeks to provide for BMD development decisions. According to this figure, SDIO does not see complete understanding of later system phases as prerequisite to initial commitments to develop and deploy BMD. Instead, it proposes to seek a partial understanding of the issues surrounding the follow-on phase and provide reasonable estimates that the necessary systems could be available as needed. SDIO has affirmed the so-called Nitze criteria as requirements for the BMD options it offers: that the defenses be militarily effective, adequately survivable, and cost-effective at the margin, that is, able to maintain their defensive capabilities more easily than countermeasures could be taken to try to defeat them. 5 SDIOop. cit., footnote 2, p. IV-3, Figure 1-1. The Path to Thoroughly Reliable Defenses Strategic Defense Initiative Organization, Report to the Congress on the Stzategicllefense Im thtive (Washington, DC: April 1987), p Lt. General James Abraharnson, personal communication to OTA staff, July 7, Ronald Reagan, televised speech, Mar. 23, Time SOURCE: Department of Defense, Strategic Defense Initiative.

15 7 Figure 1-2. Development Decision Content Figure 1.3. Mission Effectiveness Improves With Phased Deployment - Phase 3 Mutually assured survival estimates Time SOURCE: Department of Defense, Strategic Defense Initiative Now Major Political Decision First of Several SOURCE Department of Defense, Strategic Defense Initiative. The SDIO has identified three phases of BMD deployments that might extend from the mid-1990s well into the 21st century (see figure 1-3). In mid-1987, SDIO proposed to proceed with a series of technology validation experiments to build and test hardware that might demonstrate the feasibility of components of a first-phase system. These experiments would require SDI budgets substantially above the levels appropriated by Congress in the first 4 years of the SDI. In deciding about funding and directing the SDI program, then, Congress must decide whether to accept, modify, or reject the phased research and deployment scenario proposed by SDIO. Options for Congress include: accept the SDIO phasing scenario and plan now to decide in the early 1990s whether the full-scale engineering development of a first-phase system is feasible or attractive, but with only a reasonable estimate at that time of whether the second and third phases would later prove feasible; such a decision would imply an intention to deploy the first phase in the mid-1990s while beginning fill-scale development of the second phase, but the actual mid-1990s decisions would depend on the progress made; decide soon to begin immediately to develop whatever technologies may be available for deployment in the early 1990s, bearing in mind that space-based weapons are, in any case, unlikely to be deployable in quantity until 1995 or beyond; plan to delay a decision on a first phase of development and deployment until advanced research confirms that the second and third phases would be feasible; return to the pre-sdi BMD research program intended to hedge against technological surprise and to deter Soviet BMD deployment, but not intended to work toward a specific deployment scenario; or add to the previous option a new emphasis on terminal defense systems designed specifically to protect elements of U.S. strategic nuclear retaliatory forces. Nature of This Report To assist Congress in making these choices, this report surveys the technologies under research in the SD I and reports, as of early 1988: which technologies might be available for each of the projected deployment phases; what is known and what remains to be learned about the feasibility of develop-

16 8 ing those technologies and manufacturing and deploying weapons based on them; what can now be said about how survivable against enemy attack space-based BMD systems themselves may be; and what can now be said about the feasibility of producing the computer software of the requisite performance and dependability. Most experts would agree that the technical issues for BMD present severe challenges. Thus, in attempting to provide the above information, this report identifies numerous demanding technical problems. The technical challenges to the SDI have been variously interpreted: From the point of view of SDI officials and contractors, questions of feasibility are challenges that the application of sufficient time and resources can overcome. They are working on most, if not all, the issues identified in this report. In another view, the obstacles to effective BMD are great, and may not be overcome for several decades; nevertheless, the kind of research SDIO is sponsoring will have some long-term military and economic benefits for the United States whatever the SDI outcome. In addition research on BMD is necessary to avoid technological surprise and to hedge against Soviet breakout from the Anti-Ballistic Missile (ABM) Treaty. From a third point of view, the obstacles to accomplishment of the SDI s ultimate goals are so complex and so great that SDIO s goals are simply implausible. Therefore, although the United States should conduct some BMD research to avoid technological surprise and to hedge against Soviet break out from the ABM Treaty, research needed for other military or civilian purposes should be carried out under other auspices. OTA attempts in this report to present realistically the available evidence about SDI feasibility. The reader must decide how optimistic or pessimistic the evidence should lead one to be and which approach to BMD research would be best for the nation. This summary organizes OTA s findings around the kinds of system designs, or architectures, for the three phases that SDIO has recently been studying and discussing. It should be recognized, however, that, except for the first phase, these architectures are illustrative, not definitive. They provide a means of thinking about and understanding how various BMD technologies might be integrated into working systems and in what timeframes. Only the first represents SDIO s proposal for actual systems to develop and deploy. Table 1-1 outlines SDIO s suggested first phase of deployment; the timeframe is strictly an OTA assessment of a very optimistic but arguably plausible period for the beginning and completion of deployments of the various elements of the system phase. Table 1-2 outlines OTA s projections of the second and third phases of BMD deployment, based on SDIO descriptions of the technologies it is researching. The overlapping timeframes ( and ) reflect - OTAassessments of very optimistic but arguably plausible periods for the beginning and completion of deployments of the various elements of each system phase. FIRST-PHASE TECHNOLOGIES AND SYSTEMS (OTA Estimates Approximately ) Goals of a First-Phase System systems that the Nation might select in the late 1980s for initial deployments in the early 1990s. OTA estimates that as a practical mat- ter given the development, manufacturing, and space transportation needs deployment In the fall of 1986 SDIO and its contractors began to study options for first-phase deployment of BMD. They attempted to design

17 9 Table 1-1. SDIO s Phase One Space. and Ground-Based BMD Architecture Component Number Description Function First phase (approximately ): Battle Management Variable Computers Boost Phase Surveillance and Tracking Satellite Several at high altitude Space-based Interceptor 100s at several 100s of Carrier Satellite km altitudes Probe or Space Surveillance and Tracking System or Space-based Interceptor Carrier Satellites Exe-atmospheric Interceptors (ERIS) 10s 10s loos SOURCE: Off Ice of Technology Assessment, s on ground-based rockets May be carried on sensor platforms, weapon platforms, or separate platforms; groundbased units may be mobile Infrared sensors Each would carry about 10 small chemical rockets or SBIs ; might carry sensors for tracking post-boost vehicles Ground-launched rocket-borne infrared sensors Satellite-borne infrared sensors Satellite-borne infrared sensors Rocket booster, hit-to-kill warhead with infrared seeker Coordinate track data; control defense assets; select strategy; select targets; command firing of weapons Detect ballistic or ASAT missile launches by observing hot rocket plumes; pass information to tracking satellites On command, launch rockets at anti-satellite weapons (attacking BMD system), boosters, possibly PBVs. Acquire RV tracks, pass on to ERIS interceptors Cued by satellite-borne or rocket-borne infrared sensors, home in on and collide with RVs in late mid-course of the systems discussed could not begin until 1995 or later and would probably take at least until the end of the 1990s to complete. The first-phase options generally exclude space-based attack on Soviet reentry vehicles in mid-course (see table l-l). While limiting the effectiveness of a BMD system, this omission eases the sensing, discrimination, and battle management tasks. Depending on the nature of the Soviet attack assumed, and depending on the effectiveness of Soviet countermeasures, the kind of system described by SDIO officials system might destroy anywhere from a few up to a modest fraction of the (now predicted number of) Soviet reentry vehicles in a full-scale attack. The SDIO has suggested such a system as only the first phase of what in the longer term would expand to a more effective system. However, the organization cites as an intermediate military purpose... denying the predictability of Soviet attack outcome and... imposing on the Soviets significant costs to restore their attack confidence. These first phases could severely restrict Soviet attack timing by denying them cross-targeting flexibility, imposing launchwindow constraints, and confounding weapon-to-target assignments, particularly of their hard-target kill capable weapons. Such results could substantially enhance the deterrence of Soviet aggression. 6 SDIO officials assert that the military effectiveness of the first-phase system would be higher than indicated by the percentages of reentry vehicles intercepted. They envisage a strategy of adaptive preferential defense. In this strategy, first the space-based layer of defense disrupts the structure of the Soviet attack. Then the ground-based layer defends only those U.S. targets of the highest value and un- Ibid., footnote 2, p

18 10 Table 1-2. OTA s Projections of Evolution of Ground- and Space-Based BMD Architecture Component Number Description Function Second phase (approximately ) replace first-phase components and add: Airborne Optical System (AOS) Ground-based Radars High Endo-atmospheric Interceptors Space Surveillance and Tracking Satellite (SSTS) Space-based interceptor Carrier Space-based Neutral Particle Beam (NPB) Detector Satellites s in fright Infrared sensors 10s on mobile platforms 1000s at few 1000s of km. 1000s at 100s of km altitudes 10s to 100s at aititude simiiar to SSTS 100s around particle beam altitudes X-band imaging radar Rocket with infrared seeker, nom nuclear warhead High-resolution sensors; laser range-finder and/or imaging radar for finer tracking of objects; May carry battle management computers Each carries about 10 small chemical rockets or KKVS ; at low altitude; lighter and faster than in phase one Atomic particle accelerator (perturber component of interactive discrimination; additional sensor satellites may be needed) Sensors to measure neutrons or gamma rays from objects bombarded by NPB; - transmitters send data to SSTS and/or battle management computers Third phase (approximately ), replace second-phase components and add: Ground-based Lasers, 10s of ground-based Several laser beams from each Space-based Mirrors lasers; 10s of relay of several ground sites bounce SOURCE: Office of Technology Assessment, mirrors; 10s to 100s of battle mirrors off relay mirrors at high altitude, directed to targets by battle mirrors at lower altitudes Track RVS and decoys, pass information to ground battle management computers for launch of ground-based interceptors Cued by AOS, track RVS as they enter atmosphere; discriminate from decoys, pass information to ground battle managers Collide with RVS inside atmosphere, but before RV nuclear detonation could cause ground damage Track launched boosters, postboost vehicles, and ground or space-launched ASATs; Track RVs and decoys, discriminate RVs from decoys; Command firing of weapons On command, launch rockets at anti-satellite weapons (attacking BMD system), boosters, PBVs, and RVs Fire hydrogen atoms at RVs and decoys to stimulate emission of neutrons or gamma rays as discriminator Measure neutrons or gamma rays emitted from RVs: heavier objects emit measurable neutrons or gamma rays, permitting discrimination from decoys Attack boosters and PBVs der attack by the fewest reentry vehicles remaining after the winnowing by the space-based layer (see box l-a). In this way, a meaningful fraction of a large set of point targets (e.g., missile silos or command posts) might be protected. Such a strategy, however, would require successful discrimination of RVs and decoys by the first-phase system sensors a technology that remains to be proven. In addition, the Soviets could counter the strategy if they could modify their current offensive systems and deploy substantial numbers of maneuvering reentry vehicles. Figure 1-3 presents SDIO s description of how the phases of SDI deployment might satisfy a spectrum of strategic goals. In evaluating the desirability of the goal of enhancing

19 11 Box l-a. Adaptive Preferential Defense The SDIO has proposed that a first-phase ballistic missile defense system (see table l-l) employ a tactic of adaptive preferential defense. If successfully executed, this tactic could give an outnumbered defense some leverage against a large attack. Preferential defense means defending only a selected set of high-value targets out of a larger number of targets under attack, thus concentrating the defensive forces. In essence, some targets would be sacrificed to increase the chances of survival of others. Adaptive preferential defense means deciding during the course of the battle which targets to defend by adapting to the distribution of the attacking RVs (missile warheads) that survive earlier layers of defense. Of the high-value targets under attack, those with the fewest RVs coming at them are defended first. Two Layers of Defense A first-phase Strategic Defense System (SDS) would include orbiting interceptors and land-based interceptors. The orbiting interceptors would first destroy a small fraction of the rising Soviet missile boosters and post-boost vehicles. Since the SDS could not at this stage predict the targets of the Soviet missiles, the defense would not be preferential: instead, it would merely subtract at random some warheads from the Soviet attack. Even if the Soviets had initially aimed the same number of RVs at each target, some would have been filtereds out by the first layer of defense. Land-based rockets would carry other interceptors into space to destroy RVs that survived the space-based attack. Tracking sensors would determine the targets of the RVs to within several kilometers. Battle management computers would determine which high-value targets were under attack by only one RV and launch groundbased interceptors against them first, until all were covered, Then the computers would determine which targets were under attack by two RVs and assign interceptors to them, and so on. In this way, few interceptors would be wasted defending targets that would later be destroyed anyway by additional, unintercepted RVs. A Simple Example Suppose, for example, that 2000 RVs were attacking 1000 targets, with 1 RV aimed at each of 500 targets and 3 RVs aimed at each of another 500 targets. Assume that the defense had only 1000 interceptors (each with a 100 percent chance of interception). If the defense assigned interceptors randomly to 1000 of the 2000 attacking RVs, about 312 targets would be expected to survive (50 percent of those under single-rv attack and 12.5 percent of those under 3-RV attack). But if it assigned 500 interceptors to defend the targets under a single-rv attack, and then assigned 3 interceptors each to-defend the next targets, a total of 666 targets might be saved. The SDI Case Analysts for SDIO have concluded that a first-phase system applying this tactic could protect a useful fraction of selected U.S. targets against the kind of attack the Soviets are predicted to be able to carry out in the mid-1990s. Some Qualifying Considerations If feasible, an adaptive preferential defense would be suitable mainly for protecting fractions of redundant, single-aimpoint targets, such as missile silos, command posts, or other isolated military installations. Largearea, soft targets (such as cities or large military installations), would present so many potential aimpoints that defending, say, a third or a half of the aimpoints in a given area would be unlikely to assure survival of the that area. In addition, the aimpoints that could be defended would be small enough that the blast and fires from exploding nuclear weapons would affect neighboring soft target areas. Serious questions also remain about whether SDIO s proposed phase-one BMD system could, in fact, successfully execute a strategy of adaptive preferential defense. In particular, if the infrared sensors of the tracking system could not discriminate between Soviet RVs and decoys, many of the ground-launched interceptors would be wasted on decoys. And if the Soviets could deploy many maneuvering reentry vehicles during the operational period of the first-phase defense system, the targets could not be accurately predicted and defended.

20 12 deterrence by forcing modification of Soviet attack plans, Congress should also be aware of the counter-arguments to that position: Many believe that, given the awesome consequences of nuclear war for the Soviet Union as well as for the United States, deterrence does not require enhancement because the U.S. threat of nuclear retaliation is already strong enough and can be kept so with timely strategic offensive modernization. Soviet military planners already face operational uncertainties, such as the unreliability of some percentage of deployed missiles. Other, less costly, more clearly feasible, methods of complicating Soviet attack plans, such as increased mobility for U.S. strategic forces, may be available. A corresponding Soviet deployment of BMD would impose uncertainties and costs on U.S. retaliatory attack plans. The context for evaluating the goal of complicating Soviet attack plans changes, however, if one accepts the point of view that it is only the first benefit on a long-term path toward mutual assured survival. In OTA's view, figure 1-4 illustrates, somewhat more realistically than figure 1-1, the relative levels of defense capability over time to be expected from phased BMD deployments, assuming their feasibility. Whether or not initial capabilities could be sustained or improved upon depends on information not likely to be available by the early 1990s. Figure 1-4. OTA Understanding of Projected Roles of BMD Deployment Phases Time SOURCE Office of Technology Assessment, 1988 Technical Feasibility of Sensors and Weapons In a first-phase system, space-based interceptors (SBI), also known as hit-to-kill or kinetic kill vehicles, would attack missile boosters and post-boost vehicles (PBVs), but not their dispensed reentry vehicles (RVs). The only mid-course interception would be near the end of that phase of missile trajectory by ground-based, exe-atmospheric interceptors. Boost-Phase Surveillance and Tracking System (BSTS) It appears feasible to develop by the mid 1990s high altitude satellites that would tell lower altitude satellites, or possibly SBIs themselves, where to look for rising missile boosters. Complex communications links among the satellites may be necessary to avoid enemy interference. Carrier vehicles ( garages ) for space-based hit-to-kill interceptors could receive data from the BSTS and track the boosters and postboost vehicles with their own infrared sensors and laser range-finders. Space-Based Interceptors (SBI) A few hundred SBI carriers that would carry a few thousand kill vehicles (rocket interceptors) might destroy a modest fraction of Soviet missile warheads in the boost and postboost phases. Such a system might be feasible to deploy starting in the projected firstphase period, but questions of engineering and cost remain unresolved. For example, considerable miniaturization of components for propulsion, guidance, and sensors would be needed to make a rocket fast enough to reach boosting missiles and light enough to be affordably launched into space. Recent progress toward such miniaturization appears promising. Substantial testing of prototype weapons would be necessary to show system feasibility. Once these technologies were proven, the affordable mass production of rocket-carrier vehicle systems for space deployment maintenance would remain a major challenge.

21 13 Exe-atmospheric Reentry Interceptor System (ERIS) The Homing Overlay Experiment of 1984 and subsequent development work suggest that it is feasible to design a ground-launched interceptor capable of homing in on objects in space under favorable conditions. Such weapons could make up an Exe-atmospheric Reentry Interceptor System, or ERIS. More research, testing, and engineering remain to be done before the United States will know if the interceptor homing warheads can be produced cheaply enough to be affordable in large numbers. The ERIS, however, is likely to be deployable before space-based BMD interceptors. Under study are both space-based and groundlaunched infrared sensor systems and groundbased radars to direct ERIS interceptors to the vicinity of their targets. Both the satellite and ground-based systems remain to be developed, tested, and affordably produced. Upgraded versions of now existing ground-based radars might also provide initial tracking information to the interceptors. In this first-phase architecture, the ERIS would rely on radars or on passive infrared detection and tracking of potential targets. Whether or not these sensors could adequately discriminate between decoys and RVs disguised as decoys remains to be demonstrated. Without such discrimination, decoys could probably cause serious problems for this late mid-course layer of defense. Developing a decoy system like this is within Soviet capabilities. Even with good discrimination by external sensors, the homing sensor on the interceptor itself would need to find the genuine RV if it were traveling within tens of meters of other, closely spaced objects. In general, many scientists and engineers working on the SDI have agreed that such countermeasures may well be feasible for the Soviets in the near term. However, both within and outside SDIO there is some dissent on the potential type, quality, number, and deployment times of Soviet countermeasures. There is widespread agreement that much more experimentation is needed on missile penetration aids such as decoys. Very little SDI money has gone to the design, construction, and testing of penetration aids, although a full understanding of their potential and limitations would be key to developing and evaluating the effectiveness of a BMD system. Besides decoys, ERIS interceptors could face many other false targets, particularly those generated by debris from PBV activity, from intercepts made earlier in the boost phase by the SBIs, or from deliberate Soviet countermeasures. Warm objects in the field of view of the ERIS interceptor s sensors might distract it from its target RV, even if it had originally been correctly pointed toward the RV by a probe or Space Surveillance and Tracking System (SSTS) sensor. Software Feasibility In the first-phase system designs now under consideration for SDI, hundreds of satellites would have to operate automatically and, at the same time, coordinate their actions with those of other satellites. The battle management system would have to track hundreds of thousands of objects and decide when and how to attack thousands of targets with little or no human intervention. Among the most challenging software tasks for such a first-phase system would be designing programs for the largely autonomous operation of hundreds of satellites. But even for ground-based components of the system, the number of objects, the volume of space, and the brevity of time would preclude most human participation in battle management. Humans would decide at what alert status and state of activation to place the system. Once the battle began, computers would decide which weapons to use when, and against what targets. A first-phase system would have the advantage of a simpler battle management problem than that of more advanced BMD systems. In particular, the space-based segment of the system would not attempt to track and discriminate among hundreds of thousands of mid-

22 14 course objects, or to assign weapons to any of them. The distribution of SBI carrier vehicles would be so sparse that the targets within its range would not be in the range of neighboring carrier vehicles. It could, for the most part, safely shoot at a target within its own range without the risk that some other vehicle had shot at the same target. Some coordination among carrier vehicles would still be necessary because the continual relative motion of carriers and targets would leave some ambiguities about which targets were most appropriate for each carrier to fire interceptors at. Although a first-phase system would have simpler tasks than a later system, its software would still be extremely complex. The nature of software and experience with large, complex software systems, including weapon systems, together indicate that there would always be irresolvable questions about how dependable BMD software was, and also about the confidence we could place in dependability estimates. Existing large, complex software systems, such as the U.S. long-distance telephone system, have become highly dependable only after extensive operational use and modification. Extrapolating from past experience with software, it appears to OTA that the complexity of BMD, the uncertainty and changeability of the requirements it must meet, and the novelty of the technology it must control would impose a significant probability of softwareinduced catastrophic failure in the system s first real battle. The issue for SDI is the degree of confidence in the system that simulations and partial testing could provide. SDIO officials argue that such tests will permit adequate confidence and that this issue is no more serious for the SDI than for all advanced military systems developed to date. Computer simulations would play a key role in all phases of a BMD system s life cycle. Battle simulations on a scale needed to represent realistically a full battle have not yet been attempted. Whether or not sufficiently realistic simulations can be created is a hotly debated question. In particular, it is difficult for OTA to see how real-world data could be gathered to validate simulations of the phenomena that must be accounted for, such as multiple enemy missile launches, nuclear explosion-induced backgrounds, and enemy choices of countermeasures. The differences between BMD software and previous complex software that is considered dependable suggests to some experts that BMD software might never be able to pass even its peacetime tests. It should also be noted, however, that both the United States and the Soviet Union now base deterrence on an offensive nuclear delivery system that has never been operationally tested either. While the United States could not be certain that a BMD system would work as intended, the Soviets could not be certain that it would not. 7 If they had at least some reason to believe the U.S. BMD system might be effective, they might be more deterred from attacking than before. On the other hand, the United States would not want to base a major change in its nuclear strategy on a BMD system in which it had little confidence. In the case of a first-phase system, whose effect on the strategic balance would be small anyway, the risk of software-induced system failure might seem acceptable. The SDIO sees software problems as challenges to be overcome rather than as insurmountable obstacles to effective BMD. It is supporting some software research intended to address the challenges. Others argue that the limitations of software engineering technology and its relatively slow rate of improvement make it unlikely that dependable BMD software could be produced in the foreseeable future. Thus far, no new software engineering developments have appeared to contradict the latter view. Survivability of a First-Phase System The survivability of any BMD system will not be an all-or-nothing quality. The question 7 Unless they had high confidence in the potential effectiveness of a secretly deployed countermeasure (perhaps a software bug planted by a saboteur programmer).

23 15 will be whether enough of a system s assets would survive for it to carry out its mission. The issue would then turn on whether the defense could make attacking the BMD system too costly for the offense, or whether the offense could make defending the BMD system to costly for the defense. (On the other hand, if the United States and the Soviet Union agreed to coordinate offensive weapon reductions and defensive deployments, they might do much to ameliorate BMD survivability problems.) To protect satellites, the defense might employ combinations of such techniques as evasive maneuver, tracking denial, mechanical shielding, radiation hardening, electronic and optical countermeasures, and shoot-back. Categorical statements that these techniques will or will not make any BMD system adequately and affordable survivable are not credible. Judgments on specific cases would depend on the details of entire offensive and defensive systems and estimates of the techniques and tactics that the opponent would employ. Space Mines A space mine is a satellite that would trail another satellite and explode lethally either on command or when itself attacked. Space mines may or may not prove a viable threat to spacebased BMD systems. Although nuclear space mines would be a very stressing threat, much more analysis would be needed to clarify the question of the viability of space mines. After repeated attempts to locate such analysis within the SDIO or among its contractors, OTA concludes that it has not yet been adequately performed. Anti-Satellite Weapons (ASATs) There is widespread agreement among experts on Soviet military practices that the initial Soviet response to U.S. BMD deployments would not be to try to develop and deploy systems based on similar technology. They would instead attempt a variety of less sophisticated countermeasures. These might include extensions of their current co-orbital, pellet-warhead anti-satellite weapon (A SAT), or else agroundlaunched nuclear-armed ASAT (or DANASAT, for Direct Ascent Nuclear Anti-satellite weapon). The susceptibility of a BMD satellite system to degradation by DANASAT attack would depend on many complex factors, including: the maneuvering and decoying capabilities and the structural hardness of the BMD satellites; the precision and reaction time of Soviet space surveillance satellites; and the speed, numbers, decoying capabilities, and warhead power of the DANASATs. Depending on target hardness, the radius of lethality of a nuclear warhead could be so great that the ASATs might need only inertial guidance (they need not home in on or be externally guided to the BMD asset). Thus they would not be susceptible to electronic countermeasures against homing sensors or command guidance systems. It appears that, at practical levels, maneuvering or radiation shielding of low-altitude satellites would not suffice against plausible numbers of rapidly ascending nuclear ASATs. There appears to be no technical reason why the Soviets, by the mid-1990s, could not deploy DANASATs with multiple decoys among the nuclear warheads. Multiple decoys would likely exhaust the ability of the defenders to shoot back at the attack unless extremely rapid discrimination of decoys and warheads were possible. It would be difficult to deny tracking of or to decoy near-earth satellites, especially large sensor platforms, if they were subjected to long periods of surveillance. If deployed while the satellites were under attack, satellite decoys would frequently not have time to lure DANASATs far enough away from the real targets. If several SSTS satellites were a key element of a first-phase BMD system, they would be the most vulnerable elements. Otherwise, the most vulnerable elements of a first-phase BMD system would be the carrier vehicle satellites for the interceptors. The carrier vehicles, or CVs, as well as sensor satellites (BSTS and

24 16 SSTS) might employ combinations of various defense mechanisms against the ASAT threat. The SDIO argues that such combinations of measures potentially offer a high degree of survivability to space-based BMD system components. For the near-term, however, no prototypes exist for carrier vehicles with these characteristics; the issue for SDI is whether in the 1990s such satellites could be developed, produced, and deployed. The Soviets, on the other hand, have already demonstrated the ability to field DANASATs by deploying rapidly accelerating, nuclear-armed anti-ballistic missiles near Moscow over 15 years ago and recently upgrading that system. Newer ballistic missiles, relying on mature technology, might also be adapted to this purpose. More advanced DANASATs appear feasible for the Soviets by the mid-1990s. DANASATs would be a stressing threat against first-phase BMD systems and could probably degrade severely the performance of such systems. The SDIO argues, however, that strong survivability measures in the defensive system could successfully counter this threat. The Soviets might also consider gradual attrition of the system in peacetime. They might use co-orbital, non-nuclear ASATs or ground-based laser ASAT weapons to take potshots at the carrier vehicles. Attack During Deployment Should the Soviets deem U.S. space-based BMD deployments to be sufficiently threatening to their national security, they might resort to attack before the system was fully deployed. Whether they waited for full deployment or not, in the first-phase architecture SBI carrier vehicles would be so sparse that they would probably have only limited abilities to help defend one another, although each might to some extent defend itself. Other survivability measures, however, might offer some protection. Attacks on Ground-Launched Systems Insofar as the ERIS ground-launched interceptor relied on fixed, ground-based early warning radars for launch-commit information, its effectiveness could be greatly reduced by nuclear or jamming attacks on those radars. Use of Comparable Technologies Responses to threats from comparable Soviet weapon systems have not been defined by the SDIO or its contractors. Indeed, a working assumption of SDIO research and analysis has been that the United States could and would maintain a consistent lead over the Soviet Union in BMD technologies for the indefinite future. Because the Soviets lag in some of the technologies required for a space-based BMD system, it seems unlikely that they would attempt to deploy SBIS for BMD in the 1990s. A more attractive option for them might be to deploy kinetic-kill vehicles as a defense suppression system rather than as a BMD system a less difficult task. They could then choose orbital configurations designed to give their weapons temporary local numerical advantages over the U.S. BMD system. In a shoot-out between the systems, at a time of their choosing, the Soviets might then eliminate or exhaust those SBI carrier vehicles within range of a Soviet ICBM launch salvo. Effective non-nuclear ASATs would, however, require good space surveillance capabilities. If a BMD system were to cohabit space with a competent defense suppression system (possibly embodying a lower technical capability), the side that struck first might eliminate the other. The fact that a lower level of technology would be needed for defense suppression than for BMD could drive a race to control access to space as soon as possible. For example, U.S. space-based ASATs might be needed to prevent Soviet ASAT deployments that could in turn interfere with U.S. BMD deployments.

25 SECOND-PHASE TECHNOLOGIES AND SYSTEMS (OTA Estimates Approximately ) Goals The goal of a phase-two system would be to enhance deterrence, first by imposing uncertainty on Soviet strategic attack plans, then by denying the Soviets the ability to destroy militarily significant portions of important sets of targets (such as missile silos or command and control nodes) in the United States. As a result, the Soviets would retain the ability to inflict massive damage on the U.S. economy and population, but would lack the ability to accomplish certain precise military objectives. At least, such denial should decrease whatever incentives may now exist for the Soviets to commit nuclear aggression (though analysts disagree on whether such incentives do now exist); at best, the Soviets might be induced to negotiate away their militarily obsolescent missiles. If the Soviets believed they could restore their compromised military capabilities at an acceptable price, they might attempt to do so by adding new offensive weapons and by attempting both active and passive countermeasures against the U.S. BMD system. Even if they did not believe they could recapture lost military capabilities, but only believed that they were in danger of losing any credible nuclear retaliatory power against the United States, they might still attempt to employ BMD countermeasures. If, however, they concluded that countermeasures would be futile, they might, as conjectured in the SDI scenario, agree to mutual offensive arms reductions as a way of containing the U.S. threat. In that case, BMD combined with effective air defenses might offer much higher levels of protection of military and even civilian targets. Currently available BMD technology for nuclear-armed, ground-based interceptors would probably allow the United States to build a system that could deny the Soviets confidence in destroying substantial fractions of certain sets of hardened or mobile targets. 8 An SDI phase-one, non-nuclear system may also be able to provide such protection. This is more likely to be the case if the defense could be configured to defend subsets of targets preferentially, and in such a way that the Soviets could not detect which targets were defended more heavily. Moreover, if the Soviets continued to aim weapons at highly defended targets, they would have fewer weapons left over to aim at softer military and civilian targets. There is less evidence that the United States could deny the Soviets the ability to strike with high confidence at many other kinds of militarily valuable, but more vulnerable, targets. There are, however, many ideas and some promising technologies for pursuing this goal. Achieving the strategic goals of this kind of system implies air defenses of comparable potential. Otherwise, except for the most urgent targets, the Soviets could shift strategic missions from ballistic to cruise missiles. Technical Feasibility Airborne Optical System (AOS) An airborne infrared sensor system would tell ground-based radars where to look for reentering objects. Such a system appears technically feasible during the 1990s. The infrared sensors, however, might be subject to confusion by high-altitude light-scattering ice crystals created as debris reentered the atmosphere, or by nuclear detonations intended to blind the system. Ground-Based Radar (GBR) Imaging radar systems would observe lighter decoys slowing down more quickly than gen- See U.S. Congress, Office of Technology Assessment, Balh%tic Missile Defense 7 echno]o@ es, OTA-ISC-254 (Washington, DC: U.S. Government Printing Office, September 1985), pp

26 18 uine RVs.Computers using this information would launch very high acceleration rockets (HEDI) with infrared homing sensors toward the RVs Tests to date indicate that such radars are feasible, but unresolved questions include their susceptibility to interference from nuclear burst, to jamming by radio-frequency jammers on incoming warheads, to signalprocessing overloads created by many simultaneously reentering objects, and to deception by carefully designed RV s and decoys. High Endo-atmospheric Interceptor (HEDI) A rocket-borne high endo-atmospheric defense interceptor would attack incoming RVs after they had begun to reenter the atmosphere. Because the rising interceptor s friction with the atmosphere would cause it to heat up, a cooled crystal window would have to protect its homing sensor. Experiments suggest that such windows are feasible, although researchers have not yet established whether they could be rapidly mass-produced. Because the HEDI would have a limited divert capability, the sensor system would need to give it a very accurate target track. A relatively short-range ground-based radar, using the upper atmosphere as a discriminant against decoys, might be the easiest way to provide such a track. This tracking method, however, would restrict each interceptor to protecting a relatively small area. Intensive coverage of all U.S. territory would demand too many thousands of missiles. Instead, the HEDI mission would be to mop up small numbers of warheads leaking through the earlier defensive layers. Thus the most useful mission for HEDI might be to protect specific, localized targets, such as ICBM silos. SDIO officials point out, however, that passive infrared sensors or long-range radars may be able to discriminate between RVs and decoys in space. Then the High Endo-Atmospheric Interceptor could be committed earlier and thus defend a much larger area. Nevertheless, in order to avoid the impression of providing a defense designed primarily to protect hardened strategic targets, rather than U.S. territory in general, the SDIO elected to omit the HEDI and its associated sensors (AOS and a terminal imaging radar or TIR) from its proposals for a first-phase BMD system. 9 Technically, however, initial deployments in the late 1990s period appear plausible. SSTS and RV/Decoy Discrimination A phase-two system would add to the firstphase architecture dozens of space-based sensors that could accurately track thousands of RVs and decoys from the moment of their deployment from the PBVs. Such sensors would require electro-optical focal planes of unprecedented size, or high-resolution laser radar systems, and considerable signal processing ability. It seems likely that, by the time a substantial U.S. BMD system could be in place, the Soviets could deploy many reentry vehicle decoys and RVs disguised as decoys. Unless these RVs and decoys could be destroyed on their boosters and post-boost vehicles, some means of distinguishing between them would have to be developed. Otherwise, the defense s ammunition would be quickly exhausted. In the terminal, endo-atmospheric phase of interception, the atmosphere might filter out all but the heaviest and most sophisticated decoys. But too many reentering objects might overwhelm local defensive sensors and weapons. In sum, effective discrimination in the mid-course of ballistic missile trajectories would be necessary to a highly effective BMD system. One proposed technique for RV/decoy discrimination is a laser radar system that might observe the movements of RVs and decoys as, or after, they were dispensed from PBVs. Subtle differences in the behaviors of the less massive decoys might give them away. Concealing deployments off PBVs or other tactics might counter this technique, but much research both on decoy technologies and spaceborne laser radars will be needed to judge the potential of either. Lt. General James Abrahamson, personal communication to OTA staff, July 7, 1987.

27 19 Various methods of passive and active discrimination have been suggested, including multiple wavelength infrared sensors, laser radar, and microwave radar. But if the Soviets could build sufficiently sophisticated decoys, differentiating decoys and RVs might be impossible without some means of externally perturbing all the objects being tracked and observing differences in how they react to such perturbations. This technique is known as interactive discrimination. So far there is no proven candidate system for the task of interactive discrimination. The program receiving the most funding has been the neutral particle beam (NPB). In this concept, a space-based atomic accelerator would fire high-energy neutral hydrogen or deuterium atoms at suspect objects. A sensor would then detect the neutrons or gamma rays emitted from heavier objects struck by the hydrogen atoms. A hundred or more NPB platforms, and perhaps several hundred sensor satellites, would be needed for a complete system. It may be more appropriate to consider such a system for a phase-three, rather than phase-two, BMD architecture. A space test of a subscale NPB platform was scheduled for the early 1990s, although recent budget cutbacks have made the experiment s status unclear. Key issues determining the feasibility of NPB systems will include cost, the rapid and precise ability to point the beams at thousands of objects in a few tens of minutes, and the ability to gather and correlate the return information. Other interactive discrimination ideas include, for example, space-based high energy lasers that would tap target objects. The greater recoil of lightweight decoys would give them away. Kinetic Energy Weapons Missile boosters that completed their boost phase in about 120 to 140 seconds slightly faster than current modern ICBMs would greatly reduce the effectiveness of rocketpropelled SBIs in the boost phase. They could still intercept post-boost vehicles. However, fast RV dispensing technologies could reduce kill in the post-boost phase. On the other hand, if such countermeasures had forced the Soviets to greatly reduce missile payloads, mid-course discrimination might become easier: then the Soviets could only afford to deploy fewer, less sophisticated decoys. Improved SBIs, even though ineffective against boosters, could be useful in the mid-course. They would require long-wave infrared sensors for homing in on small, cold RVs. Alternatively, laser designators on sensor satellites might illuminate RVs with light that SBI sensors could see and track. It seems likely that by roughly the period projected for the first phase ERIS (Exe-atmospheric Reentry Interceptor System) missiles could be refined to the specifications now envisioned. Provided that the challenge of RVdecoy discrimination had been overcome, they would begin to provide an important layer of missile defense. If the discrimination problem could not be solved, ERIS interceptors would be of doubtful utility. If it could be solved, ERIS effectiveness in phase two would be much greater than in phase one. The question for HEDI in the phase-two period is whether the Soviets could deploy many maneuvering reentry vehicles to evade the system and sophisticated reentry decoys to deceive it. The more effective the earlier defensive layers might be, the less the Soviets could afford to use precious missile payload weights on heavier RVs and decoys. However, numerous, even slightly, maneuvering reentry vehicles, especially with depressed missile trajectories, could probably evade HEDIs unless the interceptors were equipped with nuclear warheads. Software Feasibility A phase-two BMD system such as envisaged here would need to account for hundreds of thousands (or more) of objects as they were dispensed into space. It would require a highly complex communications net for keeping track of all BMD space assets, boosters, PBVs, RVs, decoys, and space debris, then assigning weapons to intercept the selected targets. Concepts,

28 20 but so far no genuine designs, exist for partitioning the battle space into local networks of sensors and weapons (taking into account that different combinations of satellites would be constantly shifting in and out of given regions of space). In terms of sheer computing power, continued advances seem likely to provide the processing capacities needed for advanced BMD. The most difficult hardware engineering task will be to combine the qualities of high capacity and radiation hardness in space-qualified electronics. A BMD designed for boost, post-boost, midcourse, and terminal battle is likely to be the most complex system ever constructed. In OTA s judgment, there would be no precedents for estimating the likelihood of the BMD software system s working dependably the first time it was used in a real battle. Moreover, no adequate models for the development, production, test, and maintenance of software on the scale needed currently exist. The system s complexity, coupled with the need to automate the use of technologies previously unused in battle, might result in unforeseen problems dominating the software life cycle. For example, large, complex systems that undergo continuous change sometimes reach states where new changes introduce errors at a greater rate than they remove errors. A BMD system as has been the case with other strategic nuclear systems could be tested only with computer simulations and some piecemeal hardware exercises. Furthermore, no existing systems must operate autonomously (without human intervention) in the face of deliberate enemy attempts to destroy them. Whether the risks of catastrophic BMD failure resulting from the inevitable software errors in a system of this magnitude would be unacceptable is a policy decision, not a technical one, that the President and the Congress would ultimately have to make. They would have to weigh those risks against the perceived risks and benefits of not building a BMD system but deploying national resources elsewhere. As with a first-phase system, another consideration would be the likelihood that the Soviets could not be confident that the BMD system would not work as advertised, and that they might be deterred from trying to find out by attacking. (On the other hand, if the Soviets found away to break into and tamper with the software system without U.S. knowledge, they might be confident that they could defeat it.) Phase-Two Survivability More advanced BMD systems would be designed and deployed with more advanced selfprotection or survivability measures. Groundlaunched, nuclear-armed ASATs (DANASATs) would continue to be a threat. The additional SBI carriers available after the year 2000, however, could begin to provide mutual defense for one another, which would not be possible in the first-phase architecture. By that time, on the other hand, the Soviets could develop more advanced anti-satellite weapons and space surveillance sensor systems. Most BMD weapon technologies for use in space or against targets in space are likely to achieve ASAT capabilities before they become applicable to BMD missions. Direct-Ascent Nuclear ASATs As with phase one, DANASATs would be particularly threatening to a phase-two system. The U.S. Space Surveillance and Tracking System and any associated interactive discrimination platforms would now be primary targets for Soviet defense suppression attacks. Since many of these satellites would be at higher altitudes than the SBI garages, they would have more time to maneuver away from attackers. But they would also be heavier and therefore more fuel-costly to maneuver. They would be more difficult to shield against nuclear radiation. Space Mines The United States would have to consider the possibility of Soviet attempts to co-orbit nuclear or non-nuclear space mines with these

29 21 platforms as they were being deployed. Such mining might be carried concurrently with the deployment of the BMD system assets. System designers have proposed keep-out zones to keep potential attacking weapons outside their lethal ranges. Whether the United States (or any power) could achieve this kind of dominance of near-earth space remains to be seen. In any case, very little analysis has as yet been carried out by the SDIO or its contractors on interim and long-term space-based threats to BMD systems. Comparable Technologies If the Soviets could develop technologies comparable to those of the United States, three might be of special concern. One would be advanced space-based surveillance systems permitting better-timed, more accurate ASAT attacks. Second would be the development of space-based neutral particle beam weapons, which could be very effective anti-satellite weapons from great range. Third, even though laser weapons might not have achieved the power levels necessary for the BMD missions, laser ASATs could begin to pose substantial threats to U.S. space assets. If only for selfdefense, the United States might have to consider deploying directed-energy ASATs in the phase-two architecture period. THIRD-PHASE TECHNOLOGIES AND SYSTEMS (OTA Estimates Approximately ) Goals In the SDI scenario, the first goal of a phasethree BMD system would be to sustain the capabilities of the second-phase system as more advanced Soviet countermeasures came on line. Eventually, the system might achieve still higher levels of protection. As originally presented by the Administration, the SD I was to identify a path to the assured survival of the U.S. population against nuclear attack. An intermediate step on this path would be to design a BMD system that would make nuclear ballistic missiles impotent and obsolete. In this scenario, the Soviets would then be confronted with the choice of negotiating away obsolescent missiles or engaging in a costly defensive-offensive arms race that would sooner or later leave their offensive missiles unable to penetrate U.S. or allied territory. Either way, in the end few or no nuclear ballistic missiles could reach U.S. territory. 10 O SDIO reports to Congress make no mention of assured survival, and cite as the ultimate objective of the SDI to secure a defense-dominated strategic environment in which the U.S. and its allies can deny to any aggressor the military utility of ballistic missile attack. SDIO, op. cit., footnote2,p.ii-11. Other SDIO documents, however, do still refer to the goal of mutually assured survival (see figure 1-3). As with a second-phase system, extremely effective air defenses would be an essential complement to an extremely effective BMD system. And, as with earlier phases, deep reductions in offensive forces (by arms control agreement) could increase the effectiveness of the system. Technical Feasibility Directed-Energy Weapons Directed-energy weapons for boost-phase interception are still far in the future. It is unlikely that confidence in their feasibility could be established by the early 1990s even with requested SDIO budgets. OTA judges that experimental evidence of the feasibility of BMD directed-energy weapons (DEW) is at least a decade away. 11 It is extremely unlikely that confidence in DE W could be established in the next several years, given continuation of the actual appropriation pattern. 1 A similar conclusion was reached by a committee of the American Physical Society in Science and Technology of Directed Energy Weapons: Report of the American Physical Society Study Group (April 1987), p. 2.

30 22 Ultimately however, directed-energy weapons may be necessary to intercept long-range ballistic missiles and direct-ascent ASAT weapons in the boost and post-boost phases. If the Soviets could, over 15 or 20 years, develop and begin to deploy very fast-burn, laserhardened boosters with single (or few) warheads (and associated decoys) and if they deployed those boosters at concentrated launch sites, the burden even on directed-energy weapons would be great. In that case, the time available for attacking each booster might be so short as to drive very high the requirements for power levels, retargeting speed, and numbers of directed-energy weapons. (However, PBVs would continue to be vulnerable to DEWs.) Fast-burning Soviet boosters appear technically plausible the main issue would be cost. The Soviets would have to deploy enough of these boosters to continue to deliver hundreds of thousands of RV decoys into the mid-course, and they would have to be aware that, for example, if U.S. DEWs achieved significant improvements in retargeting time, they might neutralize a good fraction of the Soviets expensive fast-burning fleet. Although some work has continued on chemical lasers, and proposed future budgets would increase the share going to them, most SDI laser funding in 1987 went to the free electron laser (FEL). The most likely way to deploy such lasers would be on the ground, with orbiting relay and battle mirrors to focus laser beams on Soviet boosters and PBVs. Scientists have made significant progress in FEL research, but they are a long way from having established the feasibility of a weapon. The SDIO has sponsored construction of laboratory versions of FELs and plans a major test facility at White Sands Missile Range. Among the outstanding issues to be studied with these experimental lasers are whether FELs can be made bright enough at useful wavelengths and the feasibility of optical techniques for successfully passing very high energy laser beams out of and back into the atmosphere. Other outstanding issues include: whether large, agile beam directing optics can be affordably manufactured and reliably based in space; the cost of building and maintaining several large laser ground station complexes; and the survivability of space mirrors and ground stations against defense suppression attacks. Other directed-energy concepts are under consideration. Neutral particle beams (NPBs), which do not penetrate the atmosphere, might engage those missile boosters and PBVs that operated above about 120 kilometers. Advanced booster and warhead dispensing technologies, however, might evade NPBs. (Unlike most lasers, however, NPBs could penetrate and destroy reentry vehicles in the mid-course.) Another directed-energy weapon may be the nuclear-explosion pumped x-ray laser, which also could not penetrate far into the atmosphere. For various reasons, the x-ray laser appears more promising as an anti-satellite weapon than as an anti-missile weapon. Software Feasibility If an interactive discrimination system were added in the phase-two architecture, the phasethree architecture would not pose significantly different software challenges and prospects from the second phase. The very fine pointing and tracking needed for laser weapons could impose significant additional computing requirements on sensors. As time went on, Soviet defense suppression threats-weapons aimed at the BMD system itself-could grow more intense. The additional burdens of self-defense for the BMD system against advanced ASAT threats would add to the complexity of software requirements. The challenges to producing dependable software cited above would persist in phase three. Phase-Three Survivability If large directed-energy weapon platforms were deployed in space (whether these were laser generators with beam directors or only relay and battle mirrors for ground-based lasers), they would themselves become prime highvalue targets for defense suppression attacks. Unless they were powerful enough to be de-

31 23 ployed at rather high altitudes, they would have a difficult time either denying tracking to enemy sensors or maneuvering out of the way of attacks. They would probably have to defend themselves (and one another) as well as depend on escort interceptors. Thirdphase directed-energy weapons systems could be survivable against the current or first-phase Soviet DANASAT threat; the question is, would they be survivable against a later DANASAT threat that might be in place by the time the directed-energy weapons were deployed? Directed-Energy ASATs Long before directed-energy weapons such as lasers or particle beams achieve the capabilities they would need as BMD weapons, they could be effective anti-satellite weapons. Antisatellite laser weapons, if placed in space before more capable BMD laser weapons, might successfully attack the latter as they were being deployed. In some cases, such as the nuclear bombpumped x-ray laser, the most likely application of an advanced directed-energy weapon would be as an ASAT. What little analysis has been done so far indicates that x-ray laser ASATs launched from the ground to fire from the upper atmosphere would be difficult, if not impossible, to counter. However, the feasibility of x-ray laser weapons remains to be demonstrated. Soviet Possession of Comparable Technologies As one attempts to project various combinations of survivability techniques and various modes of anti-satellite attack into the far term, the situation becomes even hazier. It does appear that two DEW ballistic missile defense systems occupying space could pose risks of crisis instability. The side that struck first in a simultaneous attack on all the other s DEWS might seize an advantage. Much would depend on each side s tactics and its ability to jam, spoof, or disable the sensors on the other side. At best, each side might neutralize the other s BMD system, leaving both defenseless but with nuclear retaliatory capabilities (as is the case today). At worst, the side striking first might unilaterally neutralize the other s BMD (and other military space assets), leaving him open to nuclear blackmail. Mutual fears of this possibility might lead to crisis instability. On the other hand, if the two sides could define precisely balanced deployments and rules for ensuring the mutual survivability of their systems, and then arrive at verifiable arms control agreements providing for them, they might avoid such instability. costs Some experts in space systems argue that the major cost driver of space-based BMD would be the manufacture of hundreds or thousands of novel, yet highly reliable, spacecraft. The SDIO suggests that its research into new production techniques would result in substantially reduced costs. Until such techniques have actually been demonstrated in practice, this suggestion will be difficult to verify. In any case, space transportation cost would be a major challenge. The SDIO has spoken IMPORTANT GENERAL ISSUES of ultimately requiring launch operating costs one-tenth those existing today (not counting the costs for development of such a system). For the nearer term (late 1990s) the goal appears to be a threefold operating cost reduction. For the very near term, planners are being told to design systems that could evolve into less costly ones, but there is little expectation of immediate first-phase savings. Components today are conceptual, so reliable cost estimates are not possible. Efforts to improve producibility and operations costs for SBIs, ERIS, and HEDI are also conceptual.

32 24 System architects estimates put the costs of designs comparable to the second-phase architecture in the low hundreds of billions of dollars. Given that the United States would have to engineer, build, and deploy entirely new classes of space systems, cost estimates today are shaky at best. For any given component, unanticipated difficulties might increase costs, or technical breakthroughs might decrease costs. The SDIO has produced a rough estimate for the cost of a phase-one system: $75 billion to $150 billion. Phase-three architectures are now so loosely defined and understood that few if any contractor cost estimates exist. Nobody now knows how to calculate, let alone demonstrate to the Soviets, the costexchange ratio between offense and defense. Detailed defensive system designs and a thoroughly researched understanding of potential offensive countermeasures may help. But unless the ratio appears obviously to be much greater than one-to-one, it will be extremely difficult to determine whether the criterion of cost-effectiveness at the margin has been met by any proposed BMD system. At least in the first phase, it appears that the Soviets would have a strong incentive to add missiles, warheads, and countermeasures to attempt to restore their strategic nuclear capabilities. The question would be whether the Soviets were persuaded that in the long run the defense system would evolve into one that cost less per Soviet RV destroyed. 12 Timing and Evolution The Strategic Defense Initiative Organization (SDIO) has not pursued the SDI as an open-ended research program to be concluded only when a certain level of knowledge was attained. Instead, the research has been strongly oriented toward trying to provide the basis for %s discussion does not address whether the Soviets would accept the costiexchange ratio criterion for their own decisions or whether they might simply do their best at improving their offense and hope the United States might not follow the ensuing offensive-defensive arms race through to its expensive conclusion. an informed decision on BMD full-scale engineering development by the early 1990s (the exact year, although it appears widely in the press, is classified). Nevertheless, implied in the SDI program was that whatever information might be available by the early 1990s, proposals for deployment would be offered. Congress, however, has not funded the SDI at the level that the SDIO asserted was necessary to permit an informed decision about such proposals by the early 1990s. Nevertheless, by cutting back parallel technology programs and longer-term research while preserving programs believed to have near-term promise, the SDIO has attempted to maintain the goal of making detailed deployment proposals by only 1 year later than the appointed date. In late 1986 and in 1987 the SDIO began developing the phase-one BMD system architecture described above. In its 1987 annual report to Congress, the SDIO said that its study of the first phase of a phased deployment... does not constitute a decision to deploy. Such a decision cannot be made now. 13 OTA concurs. First, the required space transportation system is unlikely to be available for early 1990s deployment. Second, the reductions in SBI weights essential to deploying significant numbers of effective weapons are not yet available. Third, the U.S. aerospace industry would have to engineer, mass produce, and deploy entire new classes of satellite systems. Fourth, cost estimates for all these steps today are shaky at best. The SDIO does argue that the first-phase option would lay the groundwork for the deployment of subsequent phases. This could be true if the subsequent phases were in fact known to be feasible, affordable, survivable, and cost-effective at the margin and if the first-phase system retained some capability against a responsive Soviet threat. Every part of the complex development, production, and deployment scheme would have to work well and on schedule. Otherwise, the Soviets could be well on the way to neutralizing the first-phase architecture before it was 13 SDIO, op. cit., foot note 2, p

33 25 fully in place. Countermeasures could have greatly degraded SBI capabilities. For example, as the booster rocket burning times of Soviet missiles decreased (a process already occurring as the Soviets move to solid-fueled boosters), fewer SBI s could reach the boosters before their post-boost vehicles had separated and begun to dispense reentry vehicles and decoys. New post-boost vehicles, which would in any case be harder to track and hit than boosters, could also dispense their payloads more rapidly. Without altering their rocket technologies, the Soviets could concentrate their ICBM bases so that fewer SBIs would be in range when many ICBMs were launched at once (that is, the absentee ratio would be higher). While the Soviets would not find all such countermeasures cheap and easy, one should compare their cost and difficulty to those of developing and deploying a vast new space-based BMD system. Adding more SBIs to the BMD constellation would allow attacks on more boosters, but the numbers of SBIs needed would become increasingly prohibitive as the Soviet ICBM force evolved. On the other hand, if the Soviets Figure 1-5. SDIO Proposal for could not soon reduce the burn-times of their post-boost vehicles, SBI effectiveness might endure for some time-assuming that the firstphase SBI infrared sensors could effectively home in on the colder PBVs. Although a phase-one architecture may be presented to Congress as the first step of a phased deployment, research on the later phases is far from demonstrating that those succeeding phases will be feasible, affordable, and compatible with first-phase systems. The feasibility of fully trustworthy battle management software systems may never be entirely demonstrable. The feasibility of directed-energy weapons and interactive discrimination systems remains to be demonstrated, and persuasive evidence one way or the other will probably not be available until after The feasibility of a new, post-2005 generation of Soviet fast-burn boosters that could stress even directed-energy weapons remains plausible and cannot be discounted. Thus a phased deployment in which only the first phase was shown to be feasible would not necessarily be able to evolve and adapt to Development and Deployment Phase 1 Strategic Defense System (SDS) Concept Development approval approval (Milestone 1) (Milestone 2) Production approval (Milestone 3) Initial Operational Capability (IOC) Phase SOURCE Office of Technology Assessment, adopted from Department of Defense information on the Strategic Defense Initiative

34 26 a responsive Soviet threat. The SDIO plan calls for completing demonstration and validation of phase-two concepts before actual production and deployment of phase one. Therefore, commitment in the early 1990s to a phaseone development would imply confidence that phases two and three will ultimately prove feasible, and commitment in the mid-1990s to phase-one deployment would require an act of faith that phase three would prove feasible. Otherwise, depending on how long deployment actually took and how effective the Soviet response was, either the first- or secondphase systems could be reduced to only modest effectiveness or impotence even before deployment was completed. SDIO officials and contractors have surmised that the technologies needed to maintain and extend the defensive capabilities of first- and second-phase systems into the farther term will in fact become available. If a continuing, vigorous research and development program produced the necessary technologies, and if Soviet offensive developments could not keep pace, the first-phase concept might evolve into a more advanced BMD system. If the Soviets responded to the SBI system by developing faster-burning PBVs that could carry only much reduced payloads, then the ultimate task of discriminating RVs and decoys in the mid-course could be greatly simplified. (This conclusion assumes that the Soviets could not afford at the same time to double the size of their missile fleet.) The United States could add sophisticated SWTS satellites and SBIs with improved sensors. If Soviet decoys were few enough and simple enough, the sensor satellites might be able to track and discriminate RVs and decoys in midcourse, thus allowing improved hit-to-kill weapons to attack RVs individually after they were dispensed. Or, interactive discrimination techniques might turn out to make RV/decoy discrimination feasible. OTA concludes that, if shown to be technically feasible and desirable, second-phase system production and deployment could not begin until around the year 2000 or be completed much before Soviet countermeasures coming into deployment by then could include more missiles, advanced RVs (possibly including maneuvering RVs or MaRVs ) and decoys, faster rocket boosters and post-boost vehicles, concentrated launch-sites for boosters, and advanced anti-satellite weapons. The utility of space-based SBIs for boost-phase interception would then be severely limited. Depending on whether and when the Soviets could field faster-dispensing PBVs, the SBIs might be of some utility for PBV interception. Overall system effectiveness, however, would probably depend heavily on how well the midcourse discrimination challenge had been met. If the Soviets developed high-payload, fastdispensing PBVs, the United States might have to add laser weapons to the defense system to increase boost- and post-boost intercepts to reduce the mid-course discrimination burden. As is noted below, however, even this step might not suffice. As of 1988, three uncertainties about the viability of a second-phase system especially stand out: evidence demonstrating effective and affordable technology for discriminating Soviet nuclear warheads from decoys will probably not be available before the mid- 1990s, if then; a follow-on, directed-energy BMD system would be needed to restore or maintain defense effectiveness once faster-burning boosters were able to evade SBIs; but directed-energy weapons for BMD mayor may not be technically feasible; such feasibility is very unlikely to have been determined by the early 1990s; if the Soviets were able to field a few thousand very fastburning boosters with one warhead and several decoys each, even directed-energy weapons might not suffice to maintain a high level of defense effectiveness; the survivability of a space-based system itself against a defense suppression attack by Soviet weapons likely to be available after the year 2000 may not have been determined by the early 1990s.

35 27 Ballistic missile defense deployments of dubious long-term effectiveness could stimulate the Soviet Union to offensive countermeasures and weapon deployments rather than to negotiations to reduce mutual offensive threats. Competition in Anti-satellite Weapons As noted above, the technologies applicable in exo-atmospheric weapons are, in most cases, liable to be applicable in ASAT weapons before they are applicable in BMD. Thus there will be pressures from the military establishments on both sides to field such weapons as they become feasible, whether or not they prove to have BMD potential. For example, the first mission for space-based SBIs maybe as defensive satellites, or DSATS, to protect the BMD system as it is being deployed. Space lasers may be attractive ASATs and DSATs whether they are adopted as BMD weapons or not. Neutral particle beam discriminators could be powerful ASAT weapons. If the nuclear-pumped x-ray laser can be developed as a weapon which is far from proven its most promising application may be as an ASAT. No credible answer to the x-ray laser as a BMD suppression weapon has been developed. As the United States or the Soviet Union began to deploy substantial numbers of BMD weapons on the ground or in space, these weapons would greatly increase the anti-satellite threat to the other s space assets. (Space-based weapons themselves would, of course, be among those space assets.) Neither side is liable to permit the other the kind of unilateral control of space that such unchallenged ASAT capabilities would provide. Therefore, in the absence of arms control agreements to the contrary, we should expect from the beginning of BMD space deployments an intense competition between the superpowers for control of near-earth space. A frequently proposed survivability measure for U.S. space-based BMD assets is the enforcement of keep-out zones against any potentially threatening Soviet satellites. Whether, when, and how the Soviets might challenge such assertions of U.S. exclusionary zones in space has not been analyzed by those proposing this tactic. Indeed, the whole question of the mutual occupation of space by weapons of comparable capability has not yet been adequately addressed by SDIO or its contractors.

36 Chapter 2 Introduction

37 CONTENTS Organization of This Report Chapter l: Summary Chapter 2: Introduction Chapter 3: Designing a Ballistic Missile Defense (BMD) System: Architecture and Trade-Off Studies Chapter 4: Status and Prospects of Ballistic Missile Defense Technologies, Part I: Sensors Chapter 5: Status and Prospects of Ballistic Missile Defense Technologies, Part II: Weapons, Power, Communication, and Space Transportation Chapter 6: System Development, Deployment, and Support Chapter 7: System Integration and Battle Management Chapter 8: Computing Technology Chapter 9: Software Chapter 10: Nondestructive Countermeasures Against Ballistic Missile Defense Chapter 11: Defense Suppression and System Survivability Chapter 12: Defense Suppression Scenarios The Goals of the Strategic Defense Initiative Three Goals for Strategic Defense Increase Attacker Uncertainty Deny Military Objectives Assured Survival The Soviet Ballistic Missile Threat Targets To Be Defended The SDI Scenario Current SDI Goals The Criteria of Feasibility Effectiveness Affordability Favorable Cost-Exchange Ratio Survivability Box Box Page 2-A. Potential Targets for a Soviet Nuclear Ballistic Missile Attack Figures Figure No. Page 2-1. Nuclear Forces-ICMBs Modernization of Soviet ICBMs Warhead Mix Table Table No. Page 2-1. Strategic Defense Initiative Scenario

38 Chapter 2 Introduction ORGANIZATION This report identifies questions to be answered before the technical feasibility of achieving the goals set for the Strategic Defense Initiative (SDI) can be determined. The report also offers a snapshot of how far researchers have come toward answering these critical questions and how much remains unknown. Chapter 1: Summary Chapter 1 summarizes and explains the principal findings of this OTA study. Chapter 2: Introduction This introductory chapter devotes considerable attention to goals for the SDI, since this subject continues to be a source of confusion and debate in the country. Various leaders in the Administration and in Congress have at one time or another emphasized different goals, and which goals will ultimately prevail remains uncertain. Clearly, some goals would be easier to reach than others. This discussion does not include a critical analysis of the goals nor does it attempt to resolve the debate about them. Instead, this chapter tries to provide a context for the issues of technical feasibility. Chapter 3: Designing a Ballistic Missile Defense (BMD) System: Architecture and Trade-off Studies To assess the feasibility of a potential BMD system, the United States needs to know both what the system s elements and the system as a whole might look like. To this end, the Strategic Defense Initiative Organization (SDIO) has awarded a series of contracts to several teams of defense companies to try to define some candidate system architectures for BMD. Drawing on these studies, SDIO syn- Note: Complete definititions of acronyms and initialisms are listed in Appendix B of this report. OF THIS REPORT thesized its own reference architecture to help SDI researchers understand the requirements that the technologies being developed eventually must meet. Late in 1986 and in the first half of 1987, system architecture analysis was in a state of flux as SDIO instructed its contractors to conceptualize the early stages of a BMD deployment. In mid-1987, the SDIO proposed a firstphase architecture to the Defense Acquisition Board and in September the Secretary of Defense approved a program of demonstration and validation for this architecture. The process of evolving system architecture analysis and design is likely to continue throughout the life of the program and into the period during which defenses are actually deployed, if they are. There should be continuing feedback between system designers and technology develpers, balancing the desirable and the possible. This chapter introduces that process, discusses its importance, and describes where it has led so far. Chapter 4: Status and Prospects of Ballistic Missile Defense Technologies, Part I: Sensors Chapter 5: Status and Prospects of Ballistic Missile Defense Technologies, Part II: Weapons, Power, Communication, and Space Transportation These chapters are organized as reference works on several of the key technologies under research in the SDI program-describing them, surveying the requirements they must ultimately meet, and reporting their status (including key unresolved issues) as of early The chapters also examine the requirements for combining those technologies into working components of a BMD system, with emphasis on the kinds of components needed for recent SDIO reference architecture formu- 31

39 32 lations. Chapter 4 reviews technologies for finding, tracking, and pointing weapons at missile boosters, post-boost vehicles, and reentry vehicles and for discriminating between genuine targets and decoys. Chapter 5 reviews the weapon technologies for delivering lethal doses of energy (kinetic or electromagnetic) to targets. It also addresses the key technologies of space transportation, communication, and power supplies for space assets. Chapter 6: System Development, Deployment, and Support If BMD is to play a role in U.S. national strategy, the technologies described in the previous chapters must be incorporated into working weapon components. Those components must be integrated into effective weapon systems that are affordable, maintainable, and adaptable over time to possible adversary responses. By focusing on some particularly challenging issues, such as the development and engineering of a space-based space surveillance system and the logistics of space transportation, chapter 6 attempts to give an appreciation of the steps involved in these processes. Chapter 7: System Integration and Battle Management With variations on SDIO s reference architecture for a BMD system as models, this chapter shows how the various components of such a system would have to work together to intercept a ballistic missile attack in its several phases. The chapter attempts to give an appreciation of the complexities of integrating BMD system components into a quickly reacting system. It does so by presenting an overview of the tasks a BMD system would have to perform and examples of how it would perform them. It also exaxnines the concept of BMD battle management and the roles of humans and computers in such a battle. Chapter 8: Computing Technology Computers would be crucial to any BMD system, from simulation testing of theoretical designs, through operation of most of the hardware, to management of the battle. Chapter 8 focuses on the roles of computers in BMD and on the computation capabilities needed to satisfy SDI requirements. Computing technology encompasses both hardware and software. This chapter, however, emphasizes hardware questions while chapter 9 focuses on software. Chapter 9: Software The legislation mandating this study instructed that it include an analysis of the feasibility of meeting SDI software requirements. Chapter 9 examines the question of whether the complex computer programs that BMD will require could be made sufficiently dependable. It analyzes the concepts of software trustworthiness and reliability, as well as other important software issues. It compares requirements and characteristics of BMD software to existing, trusted software systems. The chapter ends with conclusions about the prospects for producing trustworthy software for the SDI. NOTE: Chapters 10, 11, and 12 are now available only the classified version of this report. The descriptions here are for reference. Chapter 10: Nondestructive Countermeasures Against Ballistic Missile Defense Ballistic missile defense systems must be designed to cope with the kinds of countermeasures the Soviets might deploy against them. These include modified or new ballistic missiles, devices intended to make reentry vehicles harder to find or shoot at, and weapons that could attack the BMD system. This chapter examines the first two types of countermeasure, while chapter 11 describes the latter, or defense suppression technologies and their counters. Estimates of physically possible countermeasures must be refined by estimates of what is technically, economically, and strategically feasible for the Soviet Union. The chapter concludes with a review of the tech-

40 33 nologies that might provide responses to the potential Soviet countermeasures. Chapter 11: Defense Suppression and System Survivability The legislation instructing OTA to carry out this study placed special emphasis on the survivability of an SDI-produced BMD system in the face of an enemy attack on the system itself. The chapter reviews the technologies that might be applied to defense suppression and the technologies and tactics that might counter them. Chapter 12: Defense Suppression Scenarios In a variety of scenarios, chapter 12 identifies the most stressing attack threats that various BMD elements would be likely to face and the methods a BMD system might use to defend itself, actively or passively. THE GOALS OF THE STRATEGIC DEFENSE INITIATIVE According to the Strategic Defense Initiative Organization in 1986: The goal of the SD I is to conduct a program of vigorous research and technology development that may lead to strategic defense options that would eliminate the threat posed by ballistic missiles, and thereby: support a better basis for deterring aggression, strengthen strategic stability, and. increase the security of the United States and its Allies. The SDI seeks, therefore, to provide the technical knowledge required to support an informed decision in the early 1990s on whether or not to develop and deploy a defense of the U.S. and its Allies against ballistic missiles. What does the phrase, eliminate the threat posed by ballistic missiles, mean, and how Strategic Defense Initiative Organization, Report to the Congress on the Strategic Defense Initiative, June 1986, p. IV-1. In its 1987 report, SDIO dropped in the early 1990s from its goal; it also dropped the not from the phrase whether or not in the above quotation. might doing so enhance deterrence, stability, and security? Proponents of BMD have argued that increasing levels of defense could offer increasing benefits. Fairly modest levels of BMD, they say, might improve deterrence of a Soviet nuclear attack by increasing Soviet military planners uncertainty about the effectiveness of such an attack. Higher levels of defense capability might actually deny the Soviets even the possibility of achieving whatever military goals they might have for attack. Finally, extremely good defenses against all types of nuclear attack including attacks by ballistic missiles, cruise missiles, bombers, and other means of delivery-might essentially assure the survival of the U.S. population and society no matter what the Soviets tried to do. Then U.S. security would no longer rely on the threat of retaliation to deter a nuclear attack. SDIO officials emphasize that currently the preponderance of their attention is focused on systems and technologies intended to lead to early accomplishment of the first goal of enhancing deterrence. THREE GOALS FOR STRATEGIC DEFENSE tended targets, Soviet military planners can make some predictions about Soviet ability to destroy a chosen set of targets. Just how con- fident Soviet planners would or should be Increase Attacker Uncertainty Working with assumptions about the accuracy, explosive power, and reliability of weapons systems as well as the nature of in-

41 34 about the validity of their assumptions is extremely difficult for U.S. analysts to determine. Relatively modest amounts of strategic defense, 2 some argue, might add to the uncertainties that the potential attacker already faces. 3 He would be forced to make additional assumptions about how and which-of his warheads would be intercepted by the defenses. Insofar as a Soviet decision to launch a nuclear attack on the United States might depend on Soviet confidence in their ability to destroy a given set of targets, the protection added by modest U.S. strategic defenses might help deter such a decision. 4 Presumably, the larger factor in a Soviet decision on whether to strike first is the current high probability that a U.S. retaliatory attack would devastate much of the Soviet Union. In its 1987 report to Congress, SDIO suggested that relatively modest levels of defense might begin to add to Soviet uncertainties by denying the predictability of Soviet attack outcome... and imposing on the Soviets significant costs to restore their attack confidence. 5 There are ways the Soviets might try to reduce the uncertainties added by U.S. defenses. They might deploy offensive countermeasures designed to restore their previous level of confidence in their weapons ability to reach and destroy assigned targets. They might deploy 2 This section addresses strategic defense generically -i.e., goals for defense against all means of delivering nuclear weapons, not just against ballistic missiles. Since the SDI is directed at developing defenses only against ballistic missiles, we quickly turn to that particular task for strategic defenses. Where relevant, the report will call attention to the relationships between ballistic missile defense and other kinds of strategic defense. 3 These would include uncertainties about: the accuracy of missiles over untested trajectories; the vulnerabilities of some kinds of targets, such as command and control systems; whether the victim of the attack would launch his own missiles on warning, thus defeating the most critical objective of the attack; and the nature and results of the retaliation carried out by submarine-launched missiles, bombers, and cruise missiles that escaped the attack. 4 For a more detailed discussion of deterrent strategy, see U.S. Congress, Office of Technology Assessment, Bskh stic Missile Defense Technologies, OTA-ISC-254 (Washington, DC: U.S. Government Printing Office, September 1985), pp Strategic Defense Initiative Organization, Report to the Congress on the Strate& c Defense Im tiative, April 1987, p. II-11. additional weapons intended just to exhaust the defenses, assuring that some weapons face no defensive screen. They might attempt to circumvent the BMD system by adding more bombers and cruise missiles to their arsenal. On the other hand, the Soviets would have to make new assumptions about how well these responses would work. The Soviets might also choose to give up some weapon capabilities to preserve others: for example, some countermeasures intended to assure that a given number of nuclear warheads could penetrate the defense might be traded against sacrifices in the number, accuracy, or yield (explosive power) of those warheads. If only because the offensive task had become more complicated, at least some more uncertainty would exist than if the United States had no defenses at all. 6 Opinions vary, however, on what margin of additional uncertainty the Soviets would face and whether there might be other, less costly, and earlier ways to complicate Soviet attack problems. Deny Military Objectives Some analysts have argued that an increase in attacker uncertainty as described above is itself a sufficient enhancement of deterrence to justify deploying ballistic missile defenses. The SDIO, however, places a more rigorous requirement on defense: A defense against ballistic missiles must be able to destroy a sufficient portion of an aggressor s attacking forces to deny him the confidence that he can achieve his objectives. In doing so, the defense should have the potential to deny that aggressor the ability to destroy a militarily significant portion of the target base he wishes to attack. The goal here is not just to reduce the attacker s confidence in achieving some set of goals, but to deny him any reasonable pros- 6 Alternatively, some would argue that the Soviets might find a secret countermeasure that they were certain was capable of totally disabling the U.S. BMD system; if they combined this countermeasure with expanded offensive forces, their net certainty of attack success might be increased over what it is today. 7 Strategic Defense Initiative Organization, op. cit., p. IV-2.

42 35 pect of doing so. Suppose, for example, that the Soviets have set for their strategic forces the goal of destroying 75 percent of a particular target set. A U.S. strategic defense that could predictably allow them to destroy only 50 percent of this set would therefore deny the Soviets their goal. If the difference between the Soviets choosing to attack and refraining from attack rested on their confidence in their ability to destroy 75 percent of the targets, they would be deterred. An attack of thousands of nuclear weapons that failed in its purely military objectives, whatever they might be, would still wreak great, perhaps irreparable, damage on U.S. society. Such damage would include not only the direct effects of nuclear weapons exploding near U.S. cities, but the longer-term effects of nuclear fallout and economic and social disruption. 8 Moreover, for purposes of intimidation or deterrence, the Soviets might change their target plans to retain their ability to destroy U.S. cities intentionally. Thus we would still need to rely on the threat of retaliation to deter Soviet or other attacks (or, perhaps more to the point, threats of attack) on our economy and society. Assured Survival In his speech of March 23, 1983, inaugurating the SDI, President Reagan set an even higher goal for strategic defenses: What if free people could live secure in the knowledge that their security did not rest upon the threat of instant U.S. retaliation to deter a Soviet attack, that we could intercept and destroy strategic ballistic missiles before they reached our own soil or that of our allies? This goal goes beyond denying the Soviets an ability to destroy a militarily significant portion of some target base; it would be to 8 See, for example, U.S. Congress, Office of Technology Assessment, The Effects of Nuclear War, OTA-NS-89 (Washington, DC: U.S. Government Printing Office, May 1979), esp. ch. 4, pp Ronald Reagan, televised speech, Mar. 23, protect people. As the President said over 3 years later: Our research is aimed at finding a way of protecting people, not missiles. And that s my highest priority and will remain SO. 10 The goals of increasing attacker uncertainty, denying military objectives, and assuring national survival imply progressively more capable defensive systems, and correspondingly more difficult technical challenges. The following survey of the Soviet missile threat and the kinds of targets the United States would need to defend against that threat illustrates the scope of the strategic defense problem. The Soviet Ballistic Missile Threat The Soviets now have about 1400 intercontinental ballistic missiles (ICBMs) carrying about 6300 nuclear-armed re-entry vehicles (RVs). They also have about 944 submarine launched ballistic missiles (SLBMs) with about 2800 nuclear-armed RVs (see figure 2-l). The Soviets also have several hundred intermediate-range ballistic missiles based in the Soviet Union that can reach all or part of Europe and Asia with about 1400 nuclear RVs but these are to be eliminated under the Intermediate Nuclear Forces (INF) agreement signed in December Several hundred shorter-range missiles can deliver single warheads from tens to hundreds of kilometers; many are based in Soviet Bloc countries and can reach important targets in NATO countries. Under the terms of the INF agreement, the Soviets are also to eliminate their other missiles with ranges above 500 km. The composition of the Soviet ballistic missile force will change over the years during which BMD might be developed and deployed President Ronald Reagan, SDI: Progress and Promise, briefing in Washington, D.C. on Aug. 6, 1986, Current Policy No. 858, U.S. Department of State, Bureau of Public Affairs, Washington, DC, p. 2. Secretary of Defense Caspar Weinberger has said, When the President says that we are aiming at a strategic defense designed to protect people, that is exactly what he means. Speech at Harvard University, Sept. 5,1986, quoted by David E. Sanger, Weinberger Denies Antimissile Shift, The New York Times, Sept. 6, 1986, p. 9.

43 36 Ss SS SS SS SS SS-25 About 100 Test Center A ICBM Base Ss-N-5 39 SS-N SS-N SS-N SS-N SOURCE Test Center A SLBM/SSBN Port U.S. Department of Defense, Soviet Military Power (Washington, DC: U.S. Government Printing Office, 1987),

44 37 (see figure 2-2). The changes would be more dramatic if the Soviets attempted to counter the effectiveness of prospective U.S. defenses. Anticipating this responsive threat is a major challenge for BMD planners. The SDIO has not been assigned to address the Soviet ability, present and forecast, to deliver nuclear weapons with aircraft and ground-, sea-, or airlaunched cruise missiles. The Air Force is conducting an Air Defense Initiative (ADI) that is studying the interception of air-breating weapons. The ADI, however, is operating at much lower funding levels than the SDI. Targets To Be Defended The three goals of uncertainty, denial, and assured survival remain abstract and ambiguous until we consider the kinds of targets to be defended against nuclear attack. Soviets attack objectives might include four broad categories of targets: 1. strategic retaliatory forces ICBM silos (or, in the future, mobile ICBMs), bombers (and refueling tankers) at their bases, submarines in port, command posts, and communications nodes; 2. other military targets-including military headquarters, barracks, nuclear and con- Figure 2-2. Modernization of Soviet ICBMs Warhead Mix Ss-11, SS-13, SS-25 (mobile 1986 a Estimates based on current trends. Mid-1990s a SOURCE: U.S. Department of Defense, Soviet Military Power (Washington, DC: U.S. Government Printing Office, 1987), ventional ammunition dumps, supply depots, naval ports and shipyards, airfields, and radars; enconomic targets-industrial facilities, fuel reserves, research centers, transportation nodes, and cities; and political targets-non-military government facilities, and civil defense shelters. Each of these sets of targets (for further explanation, see box 2-A) has different implications for strategic nuclear offensive and defensive operations. Strategic Retaliatory Forces The purpose of a Soviet nuclear attack on U.S. strategic nuclear forces a so-called counterforce attack would be to reduce the ability of those forces to carry out a retaliatory nuclear attack on the Soviet Union. In 1986 the Department of Defense estimated that by attacking each of 1000 U.S. Minuteman missile silos with two SS-18 warheads, the Soviets could destroy about 65 to 80 percent of U.S. land-based ICBMs. 11 An attack would have to succeed quickly and destroy a high percentage of the targets. Otherwise, U.S. weapons could be launched against the Soviet Union (assuming they had not already been launched on warning, before the first Soviet missiles arrived). The objective of substantially reducing the retaliatory damage inflicted on the Soviet Union would not be met. Thus slower bombers and cruise missiles would be less suitable than ballistic missiles for this kind of disarming attack. * U.S. Department of Defense, Soviet Military Power 1986 (Washington, DC: U.S. Government Printing Office, 1986), p. 25. The United States maintains several hundred Poseidon and Trident missiles at sea at all times and is adding sea-launched nuclear cruise missiles to its arsenal It also maintains bombers (many with cruise missiles) on alert for rapid escape on warning. The President s Commission on Strategic Forces (the Scowcroft Commission argued in 1983 that, in view of overall U.S. retaliatm-y capabilities, ICBM vulnerability did not warrant ABM (anti-ballistic missile) defense of missile silos in the near term. Some argue that future Soviet anti-submarine warfare developments might compromise the survivabilit y of U.S. ballistic missile submarines, and that defense of land-based missiles might compensate for that eventuality. Others argue that if both the United States and the Soviet Union were to deploy BMD, U.S. retaliatory missiles would be less able to fulfill their missions, whether launched from land or sea.

45 38 Box 2-A. Potential Targets for a Soviet Nuclear Ballistic Missile Attack Strategic Retaliatory Forces Land-based ICBMs. -The United States has about 1,000 intercontinental ballistic missiles in hardened silos. In the 1990s it may deploy Midgetman missiles on road-mobile carrier vehicles. It may deploy some MX Peacekeeper ICBMs on railroad cars within U.S. military lands. An attack on land-based ICBMs would have to be swift, well-coordinated, and accurate. Otherwise, many of the missiles would remain available for striking back at the Soviets. (The Soviets would also have to consider the risk that the United States would launch its ICBMs while they were under attack, with many escaping destruction to retaliate against the Soviet Union. Bomber Bases. About 350 strategic bombers, able to carry several thousand nuclear bombs and cruise missiles, are based at some tens of airfields around the United States. Additional aircraft are needed to refuel the bombers in flight. Normally, a substantial number of the U.S. strategic aircraft are on standby alert and might be expected to escape a Soviet missile attack given several minutes of warning; in times of crisis, more bombers would be placed on alert. A Soviet attack might try to catch as many as possible of the U.S. bombers (and their refueling tankers) on the ground or just after take-off. Submarine Bases. Thirty-odd submarines with several hundred underwater-launched ballistic missiles are based at just a few U.S. ports. By plan, in peacetime somewhat more than half these submarines, with 2,500-3,000 nuclear warheads, are always at sea. Those in port would be easy, inviting targets for a Soviet strategic counterforce attack. During a crisis, some of the submarines in port could be sent to join those already at sea. Communications, Command, and Control Facilities.- Linking the above forces to U.S. National Command Authorities is a network of underground command posts, mobile command posts, mobile communications (air, ground, and space) relays, and fixed communications transmitter and receiver stations. A Soviet nuclear attack is likely to try to disrupt this network by direct nuclear destruction of the fixed land facilities or by means of nuclear-generated electromagnetic pulses intended to interfere with the functioning of electronic devices. Other Military Targets Military Headquarters; Barracks, Nuclear and ConventionalAmmunition Dumps, Supply Depots, Naval Ports and Shipyards, and Airfields. Many other military facilities, while not directly supporting U.S. rapid-response strategic nuclear forces, would be essential to the conduct of conventional warfare or tactical nuclear warfare abroad. Many of these targets are soft... difficult to shelter from the effects of even relatively inaccurate nuclear weapons. Economic Targets Factories, Power Plants, Fuel Supplies, and Transportation Nodes. -These are sometimes called economic recovery targets. The military purpose of attacking them might be to eliminate the economic base that supports U.S. military power. While the United States might be able to carry out a strategic nuclear retaliatory attack if its cities were destroyed, it could not carry on a conventional war abroad very long. Political Targets Government Facilities and Civil Defense Shelters. The Soviets might also attempt to disrupt government to hinder economic and political recovery.

46 39 The purpose of a U.S. ballistic missile defense against such an attack would be to preserve enough missiles and bombers to retaliate successfully against the targets in the Soviet Union designated by U.S. military planners. 12 At a minimum, the United States might wish defenses to add to current Soviet uncertainties about how well they could prevent those offensive weapons from reaching the Soviet Union. If these redundant, hardened targets could be defended preferentially, that is, if defensive resources could be devoted to protecting a sub-set of them that is unknown to Soviet planners, then Soviet confidence in being able to destroy the whole force might be reduced to a very low level. 13 At best, we would want defenses that persuaded the Soviet Union of the certainty of failure of any preemptive attack on our strategic forces that had the purpose of reducing significantly the damage we could do to the Soviet Union. Other Military Targets The purpose of attacking U.S. military targets other than those connected with strategic nuclear forces would be to weaken or eliminate the ability of the United States to project military power abroad (to fight conventional or limited nuclear wars in Europe, Asia, or elsewhere), or even to defend its own territory against invasion. Unlike sheltered ICBMs, most of these other military targets are relatively soft each could be easily destroyed by one or a few moderately accurate nuclear weapons. Nor must they be destroyed instantaneously, since they cannot be used for a prompt nuclear retaliation against Soviet territory. Since these other military targets can be destroyed more or less at leisure, strategic delivery vehicles other than ballistic missiles can Opinions vary greatly on how many of what kinds of targets the Soviets would have to betieve they would lose in such a retaliation before they would be deterred from launching an attack on the United States. See OTA, %Zh stic Missile Defense Technologies, op. cit., pp sfor a more detailed explanation of the concept of preferential defense, see OTA, Ballz stic Missile Defense Technolo~ es, op. cit., pp be used against them bombers and cruise missiles in particular. Therefore, a strategic defense intended to protect these targets must be highly effective against air breathing weapons as well as against ballistic missiles. The purpose of defending such targets would be to decrease the probability that a nuclear attack on them could significantly weaken our military power; at best we would want the Soviets to be certain that such an attack would fail. It is important to note that many of these other military targets are located in or near urban complexes, and an attack on them might be hard to distinguish from a punitive city attack. Fallout would reach extensive areas of the United States and millions of people might die. Urban Economic and Political Targets The main military purpose of attacking the U.S. industrial and political infrastructures would be to remove the base from which the United States exerts military and economic power abroad. Another purpose, however, might simply be to inflict punishment. Before a war occurred, the purpose of having such an ability to punish would be to deter actions (e.g., nuclear or nonnuclear attacks) by threatening to impose a cost higher than the expected gain of such actions. For example, Britain and France maintain nuclear deterrent forces that they believe help deter the Soviet Union from attacking them, even though the effects of those forces on Soviet military capabilities might be more indirect than direct. 14 Even a few tens of nuclear weapons landing on U.S. cities would cause unprecedented destruction in this country. Extensive use of civil defense measures, if feasible, might ameliorate the effects of such destruction (e.g., if city populations could be evacuated and sheltered from radioactive fallout and if industrial machinery could be sheltered). But even more so than the 141t might k now, however, that the Moscow area has mwy military facilities; attacks on them would have widespread military as well as civilian consequences.

47 40 kinds of soft military targets described above, cities are vulnerable to attacks over hours and days by bombers and cruise missiles as well as by ballistic missiles. Defending cities, then, would require extremely effective air defenses as well as missile defenses. The purpose of defending against attacks on urban industrial targets would be primarily to save lives, property, and civilized society. Militarily, the purpose of having such defenses would be to persuade potential attackers that we could so limit damage to our Nation that we would not have to constrain our own actions out of fear of the effects of an enemy nuclear attack. From the standpoint of deterrence, various considerations may affect just how much we believe we need to limit damage to our Nation. One consideration might be relative damage: would the damage the United States is likely to suffer in a nuclear war be more or less acceptable to us than the damage the Soviets are likely to suffer would be to them? Another measure might be absolute: regardless of how much damage we could inflict on the Soviets, under what conditions would we be willing to accept the amount of damage they could inflict on us (and vice-versa)? An open question is just how limited the potential damage would have to be before the United States would decide to give up entirely its own ability to carry out a nuclear retaliation against potential attackers. That is, at what point would we decide to rely on defense rather than the threat of retaliation for our own security? The Special Case of Defense of Allies Part of the stated mission of the SDI is to design defenses to protect U.S. allies against ballistic missiles. But the purposes and technical problems of doing so differ somewhat from those of defending the continental United States. In the case of North Atlantic Treaty Organization (NATO) allies, for example, the Soviet ability to deliver nuclear weapons onto Western European soil is massive and diverse. Besides their land- and sea-based long-range ballistic missiles, the Soviets might use hundreds of short-range ballistic missiles (intermediate and medium-range missiles with ranges above 500 km are to be eliminated under the terms of the INF Treaty signed in December 1987). Thousands of Soviet and Warsaw Pact tactical aircraft are credited with the ability to strike Western Europe. Air- and ground-launched cruise missiles are or will be available. The probability of being able to defend Europe s densely populated territory against all the potential kinds of nuclear attacks on cities and industries seems low. Therefore, most proponents of BMD for the European theater of war focus on the defense of what are above called other military targets -command posts, communications nodes, sheltered weapons-storage sites (nuclear and nonnuclear), and airfields. Ballistic missile defenses might at least disrupt and reduce the effectiveness of Soviet nuclear missile attacks on such targets (though other means of delivery would also need to be dealt with). Moreover, some believe that as Soviet ballistic missile accuracies increase, the Soviets might use those missiles to attack military targets with nonnuclear explosive or chemical warheads. Stopping moderately high (and in some cases even modest) percentages of the warheads in such attacks might make a military difference. 16 Others argue, however, that the conventional tactical ballistic missile threat, if it exists, is minor compared to others NATO will have to contend within the future. 16 Another mission for Soviet theater ballistic missiles might be the delivery of chemical weapons intended to incapacitate NATO troops. Again, the interception of a significant percentage of such missiles might make the difference between some troops surviving a chemical attack or not. see Manfred Woemer, A Missile Defense for NATO Europe, Strategic Review, Winter 1986, pp. 13ff. lefor a det~ed technical analysis, see Benoit Morel ~d Theodore A. Postol, A Technical Assessment of The Soviet TBM Threat to NATO, to be published by the American Academy of Arts and Sciences, Cambridge, MA.

48 47 The shorter range Soviet ballistic missiles differ in flight characteristics from their larger relatives: their trajectories are shorter and confined to lower altitudes. While they travel more slowly, their shorter flight times also leave less time for them to be intercepted. On the other hand, because these missiles spend a greater part of their flight time inside the atmosphere, reentry vehicle decoys present less of a problem to the defense. Space-based BMD (especially of the kinetic kill variety) would be of limited utility, and ground-based rocket-interceptors would be the likeliest BMD candidates. The SDI Scenario Various statements by Reagan Administration officials over the first 4 years of the Strategic Defense Initiative can be combined to form a scenario about how successively more ambitious goals for strategic defenses might be achieved. 17 The expectation of the Administration is that SDI research will show that deployment of ballistic missile defenses is feasible and desirable. As President Reagan has said, When the time has come and the research is complete, yes, we re going to deploy. In the early stages of deployment, according to the Administration scenario, Soviet attack uncertainties would increase, thus reducing the probability of a Soviet first-strike decision (though not the damage they might inflict should they choose to attack). At first, minimal defense capabilities would only complicate Soviet attack plans. As strategic defenses became more capable, the Soviets ought to be more persuaded that the military purposes of any attack would fail. Nevertheless, as long as a substantial number of targets in the United States were still vulnerable to attack, we would have to continue developing and deploying offensive strategic nuclear. _. For a list of statements prior to August, 1985, see OTA, Balh stic MissiJe Defense Technolo@ es,op. cit., App. I, pp President Ronald Reagan, SDI: Progress and Promise, briefing in Washington, DC, on Aug. 6, 1986, Current Policy No. 858, U.S. Department of State, Bureau of Public Affairs, Washington, DC, p. 2. weapons. As Secretary of Defense Weinberger has written: From the outset, we have insisted that progress toward an effective SD I will have to proceed hand in hand with regaining an effective offensive deterrent The Administration hopes, however, that ultimately offensive deterrence can be abandoned: As the United States has repeatedly made clear, we are moving toward a future of greater reliance upon strategic defense. The United States remains prepared to talk about how under what ground rules and process-we and the Soviet Union can do this cooperatively. Such strategic defenses, coupled with radical reductions inoffensive forces, would represent a safer balance and would give future statesmen the opportunity to move beyond it to the ultimate elimination of nuclear weapons from the face of the Earth. 20 The key to this ultimate goal is seen to be the development and deployment of defenses that are unequivocally cheaper than corresponding amounts of offense. As SDIO puts it: We seek defensive options as with other military systems that are able to maintain capability more easily than countermeasures could be taken to try to defeat them. This criterion is couched in terms of cost-effectiveness. However, it is much more than an economic concept Waspa.r W. Weinberger, U.S. Defense Strategy, Foreign Affairs, Spring 1986, p Earlier in the same article Weinberger explained his concept of a multi-leveled deterrent: If the adversary calculates that his aggression is likely to fail in its own terms, he will not attack. Further, he must know that even if his aggression should succeed in achieving its immediate objectives, he faces the threat of escalation to hostilities that would exact a higher cost than he is willing to pay. In addition to defense and escalation, the third layer is retaliation: if the adversary confronts a credible. threat that eg ession will trigger attacks by a surviving U.S. retaliatory capa ilit against the attacker s vltsl interests that result in losses exc &ganypossible@n, hewfl not attack. Ibid., p ~O~sident Ron~d Reagm, Speech to the U.N. General Assembly, Sept. 22, 1986, reprinted in The Washington Post, Sept. 23, 1986, p. A16. zl&ra@$c Defense Initiative Organization, tie COngress on the Strategic Defense Initiative, April 1987, p. IV-3. It should be added that not only should capability be maintainable at the margin, but that our initial acquisition of defense capability needs to be affordable in comparison with the cost to the Soviets of upgrading their current offensive capabilities to counter our defenses. The offense, being already in place, has a head start on defenses yet to be built.

49 42 Such a favorable cost-exchange ratio between defenses and offenses would be intended to persuade the Soviets of the futility of continuing a competition in offensive arms. The SDIO has stated that: Program success in meeting its goal should be measured in its ability both to counter and discourage the Soviets from continuing the growth of their offensive forces and to channel longstanding Soviet propensities for defenses toward more stabilizing and mutually beneficial ends... It could provide new and compelling incentives to the Soviet Union for serious negotiations on reductions in existing offensive nuclear arsenals. 22 Agreements on mutual offensive reductions could make defensive tasks easier for each side. Thus the Soviets could be offered both a carrot (possibility of their own effective defenses) and a stick (threat of losing an arms race between offenses and defenses) as incentives to subscribe to the U.S. scenario. Current SDI Goals The scenario shown in table 2-1 for the SDI suggests the following official attitudes toward the three goals of uncertainty, denial, and assured survival. Uncertainty Imposing greater uncertainty on Soviet attack planners would bean initial benefit of deploying BMD, but, presumably is not in itself sufficient to justify the SDI. Denial Denial of Soviet military objectives in a ballistic missile would, in itself, justify deploying BMD. Secretary Weinberger has said:... our strategic defense need not be 100 percent leakproof in order to provide an extraordinary amount of deterrence. Even a partially effective defense would convince Moscow that a first-strike was futile. And once we have rendered a Soviet first-strike obsolete and unthinkable, we will have dramatically increased ~~Ibid., pp. IV-1-2. Table 2=1. Strategic Defense Initiative Scenario Stage 1: SDI Research Stage 2: Development and production of BMD systems Stage 3: Initial BMD deployments Stage 4: Extensive deployment of highly effective BMD Stage 5: Deployment of advanced BMD systems, combined with agreed deep reductions in offenses Leads to national decision in early 1990 s to proceed to fullscale engineering development aimed at deployment of BMD (reference to early 1990s date dropped by SDIO in 1987) Preparation for deployment in mid-to-late 1990s (earlier initial deployments raised as possibility by Secretary Weinberger in 1987) Introduces uncertainty into Soviet strategic nuclear attack planning; deployments preferably coordinated by agreement with Soviets on transition to defenses, but proceeds in any case Denies Soviet strategic forces ability to achieve military objectives; demonstrates to Soviets futility of competition in offensive strategic missiles Deep reductions in all types of offensive strategic nuclear forces plus defenses allows abandonment of threat of nuclear retaliation for security: assured survival achieved SOURCE: Compiled from U.S. Department of Defense, Report to the Congress on the Strategic Defense Inltlative, June 1986, p. IV-12 and other Administration statements. stability and rested deterrence on a rock-solid basis. But bear in mind that our goal remains to make ballistic missiles-the most destabilizing and dangerous weapons known to man obsolete. 23 Assured Survival The goal of assured survival may well require Soviet cooperation in offensive nuclear disarmament. A perfect defense against all ballistic missiles may not be possible, and: Even a thoroughly reliable shield against ballistic missiles would still leave us vulnerable to other modes of delivery, or perhaps even to other devices of mass destruction. De- z~remmks before the Ethics and Public Policy Center, Washington, DC, Sept. 26, 1986.

50 43 spite an essentially leakproof missile defense, we might still be vulnerable to terrorist attacks against our cities. Our vision of SDI therefore calls for a gradual transition to effective defenses, including deep reductions in offensive nuclear weapons. 24 In the expressed Administration view, then, the SDI should aim ultimately for ballistic mis Weinberger, U.S. Defense Strategy, op. cit., p sile defense systems that are nearly leakproof. One way of achieving assured survival might be to build defenses so effective that they would succeed no matter what the Soviets might throw at them. Another way might be to build defenses that promise to be so effective that the Soviets would prefer to negotiate offenses on both sides away rather than embark on an offense-defense race that they have been persuaded they would lose technically or economically. THE CRITERIA OF FEASIBILITY Supporters and critics of the SDI would probably both agree that proposals for deploying ballistic missile defense should meet at least the four following criteria: 1. effectiveness, 2. affordability, 3. favorable cost-exchange ratio, and 4. survivability. Note that in each case, meeting the criterion will beat least partly dependent on Soviet decisions and actions: the Soviets can make the job harder or easier for the defense. In an unconstrained arms race, they would do what they could to make the job harder. In a cooperative regime of mutual defensive deployments and offensive reductions and controls, each side might make the BMD job easier for the other. Effectiveness Obviously, before deciding to deploy a BMD system we would want to be confident that it would be effective that it would work well enough to achieve the goals set for it. Effectiveness needs to be evaluated on two complementary levels. One level is technical performance: how well can the proposed BMD system perform against the missile threat expected at the time of defense deployment? On a higher level, would such performance provide abetter basis for deterrence, strengthen strategic stability, and increase U.S. and Allied security the goals stated by SD IO? This second level of analyses received considerable attention in the 1985 OTA report on Ballistic Missile Defense Technologies, so it will receive much less attention in this report. On the level of technical performance, it is difficult to decide what effectiveness means. For example, one frequently used criterion of BMD effectiveness is leakage rate : what percentage of a specified Soviet missile attack would we expect to penetrate our defenses and what percentage could we stop? Given the enormous destructive power of nuclear weapons, though, leakage rates may only tell part of the story. A leakage rate of 10 percent might sound worthwhile, and for some purposes it may be. But under an attack of 10,000 nuclear warheads, a 10 percent leakage rate would mean 1000 nuclear detonations on U.S. territory. Another problem with leakage rate as a measure of effectiveness is that it is likely to vary with the size and nature of attack. For example, a system that could stop only 50 percent of a massive, nearly instantaneous attack might stop 100 percent of an attack consisting of two or three missiles. On the other hand, a system that could stop 50 percent of an attack of a certain size might not be expandable in such a way that it could stop 50 percent of an expanded enemy missile force. In addition, to maintain damage at a fixed level, the defense would have to stop, for example, 75 percent of a doubled attack.

51 44 A slightly better indicator of effectiveness, then, might be the absolute number of nuclear warheads penetrating the defense under the severest plausible attack. Such an estimate would give abetter indication of the maximum damage a Soviet attack might inflict. An even better indicator would be the numbers of different types of targets that the United States would expect to survive a missile attack. This approach would take into account the numbers of attacking weapons, the numbers of penetrating weapons, the numbers and types of targets attacked, and the numbers and types of targets protected. These numbers might be translated into percentages of types of targets surviving-e. g., 70 percent of the land-based missile force. 25 We might carry the analysis further by weighing the values of different types of targets. For example, one underground strategic command post might be worth 10 missile silos. All of the above indicators would be difficult to apply with precision. And the more factors an indicator has to take into account, the more imprecise it is likely to be. Indeed, there would be no direct way to measure the potential effectiveness of a BMD system: only an actual nuclear war would do so. Instead, we would have to rely on estimates, based on assumptions about: enemy offensive technical capabilities (numbers of weapons, accuracy, explosive yields, ability to penetrate defenses); enemy target attack plans; defensive technical capabilities; vulnerability of targets defended; and the objective and subjective relative values of targets defended. These factors would be difficult for U.S. planners to assess. They would also be difficult for Soviet planners to estimate. Therefore, if the U.S. goal is mainly to introduce uncertainties into Soviet strategic calculations, precise measures of BMD effectiveness might not be nec- 6Note that plannin g to penetrate defenses may require the offense to concentrate his attacks on higher-value targets, In that case, the targets which he no longer has enough weapons to strike can be considered saved by the defense. essary. On the other hand, if we wished to be certain of denying Soviet attack objectives, we might need higher confidence in our estimates. At the same time, if the Soviets decided, along with the United States, that defenses were desirable, then each side could help make them more effective by agreeing to deep cuts in offensive weapons and to restrictions on countermeasures against defenses. Affordability If and when the Department of Defense eventually presents its proposals for deploying BMD, the country will have to decide whether the expected benefits would be worth the expected costs. Part of the SDI research program is to estimate costs for the proposed systems. For various reasons, the initial cost estimates for complex weapon systems tend to be inaccurate, and usually too low. Producing reliable cost estimates for future BMD systems will be a challenging task. Another part of the SDI program is to attempt to develop new, cheaper ways to manufacture weapons and to deploy them in space. 26 The ultimate weighing of costs and benefits will be a political judgment made by the President and Congress. But a critical part of the demonstration of technical feasibility of BMD will be that the proposed systems can be built at a cost the country would, at least arguably, find reasonable. As mentioned above, Soviet actions could make effective BMD more or less affordable. If they chose to invest heavily in offensive countermeasures timed to take effect about when our defenses might be deployed, they could make those defenses much more expensive than if they stabilized the threat they pose at today s levels. Alternatively, in a cooperative regime they could make defenses cheaper by agreeing to decrease their offensive threat ~Unt~ a re-organization in 1987, the SDIO Systems Endneering Directorate was in charge of this program, among others. The Systems Engineering program element of the SDI budget received $20.2 million in fiscal year 1987; $39 million was requested for fiscal year 1988.

52 45 in exchange for reductions in the U.S. offensive threat. Favorable Cost-Exchange Ratio The Nation must decide not only that a particular defense system proposed at a particular time is affordable, but whether the potential long-run competition of U.S. defenses against Soviet offenses is likely to be affordable in the future. In the absence of a long-term U.S. commitment to sustaining defensive capabilities, the Soviets would have incentives to stay in the game until the United States will to spend flagged. One way to try to persuade the Soviets to abandon efforts to maintain offensive capabilities would be to demonstrate clearly that additional increments of offense would be more costly to the Soviets than corresponding increments of defense would be to the United States. Therefore, a corollary goal of the SDI is to design defenses that are cheaper at the margin than offenses. If the cost-exchange ratio were favorable to defenses, and if the two sides invested equal resources in defenses and offenses respectively, then the side investing in offenses should find its capabilities inexorably declining. Achieving this favorable cost-exchange ratio will be technically challenging. Accurately estimating the costs of defensive systems would be difficult enough. Attaining high confidence that the ratio of U.S. defensive costs to Soviet offensive costs would be favorable, even before the United States deployed its defenses and before Soviet offensive countermeasures were known would be even more difficult. Neither side may actually know the relative costs of additional increments of defense and offense until they actually buy them. 27. * It might be argued that, faced with these uncertainties, the Soviets would accede to the U.S. proposal for a negotiated transition that regulated offensive and defensive deployments. On the other hand, drafting such an agreement that both sides would find equitable, given the asymmetries in forces and technologies on the two sides, would be a formidable task. Because the United States and the Soviet Union have such different economies, it will be difficult to quantify the cost-exchange ratio. Moreover, the effective cost-exchange ratio may differ from the technical one. That is, the ratio depends not only on what things cost, but also on what people are willing to pay. If the Soviets are willing and able to pay for an increment of offense that is more costly than our corresponding increment of defense, for practical purposes the cost-exchange ratio is at least even. The SDI objective, then, is to persuade the Soviets that the defenses we can afford will more than offset the offenses they can afford. Thus the offense/defense cost-exchange ratio may have to be not just 1.5:1 or 2:1, but several-to-one. On the other hand, if the Soviets were to agree with the United States that a mutual reduction of offensive missile capabilities was worthwhile and that defenses were desirable, then the technical challenge could be reduced. In effect, mutual political decisions could improve the cost-exchange ratio by mandating reductions rather than enhancements-of offensive capabilities, along with limitations on other offensive countermeasures. Survivability One of the many possible types of countermeasures against a BMD system is to attack the system itself-which will be called defense suppression in this report. Obviously, to carry out its defensive mission, the BMD system must survive such attacks. Survivability does not mean the ability of every element each satellite, e.g. to survive any attack. Rather, it means the ability of the system as a whole to perform acceptably despite attacks that may disable some elements. No BMD system will be either survivable or not survivable. The question will be, How survivable, at what cost? The cost-exchange ratio between defense and offense will have to be calculated on the basis of the costs of all kinds of offensive response, including defense suppression, compared to the costs of all kinds of defensive counter-countermeasures, including survivability measures.

53 46 systems that would meet the effectiveness, af- fordability, cost-exchange, and survivability criteria. The remainder of this report surveys what was and was not known as of April 1988 about the potential of the SDI for developing

54 Chapter 3 Designing a BMD System: Architecture and Trade-off Studies

55 CONTENTS The Importance of BMD Architecture Studies Overview of System Architecture Analyses Goal Specification Threat Definition System Requirements Systems Designs Technological Requirements Operational Requirements costs ! Nuclear Force Exchange Models: Deriving Requirements From Goals Some Conclusions Drawn From Nuclear Exchange Models Limitations of Nuclear Force Exchange Models System Designs and End-To-End Models Space- and Ground-Based Architectures Battle Management Architecture Some Important Results of the System Requirements and Design Work Important Systems Analysis Work Remaining Further Strategic Nuclear Force Exchange Work Further System Requirements and Design Work Tables Table No. Page 3-1. Two Perspectives on BMD Effectiveness and Strategic Goals Judgemental Assumptions in Nuclear Force Exchange Models Phases of Ballistic Missile Trajectory SDIO s Phase One Space- and Ground-Based BMD Architecture OTA s Projections of Evolution of Ground-and Space-Based BMD Architecture Two Representative Battle Management Architectures

56 Chapter 3 Designing a BMD System: Architecture and Trade-off Studies THE IMPORTANCE OF BMD ARCHITECTURE STUDIES Researchers have performed proof-of-principle experiments for some Strategic Defense Initiative (SDI) technologies. But many of the basic technologies for the SDI are still in an experimental, or even theoretical, stage. Therefore it might seem premature to be designing full-scale ballistic missile defense (BMD) systems for deployment not only in the mid-1990s, but in the 21st century. In fact, such designs are key to assessing the feasibility of achieving U.S. strategic goals through ballistic missile defense. National decisionmakers can only fully evaluate proposed systems on the merit of system architectures, not on the promise of one technology or another. If called upon to appropriate funds for BMD development and deployment, Congress will be asked to decide upon an architecture-a specific system design comprising many technologies and components. Attempting such designs, or system architectures, as the Strategic Defense Initiative Organization (SD IO) calls them, compels systematic analysis of all the factors that will affect SDI feasibility. In the near term, such analysis helps guide the technology research effort. In the long term, it will provide the substance of the national debate over whether to deploy BMD. System architecture analysis, if done well, will provide some of the key elements of information upon which to base decisions about whether to commit the Nation to deploying any proposed BMD system: Specification of Goals. Explicit identification of the particular strategic goals that BMD system designs will be expected to achieve (e.g., impose uncertainty on So- Note: Complete definitions of acronyms and initialisms are listed in Appendix B of this report. viet strategic planners); understanding of those goals in the larger context of U.S. national security; and cost-effectiveness comparisons of alternate means, if any, of achieving the goals. Specification of Threat. Projections of future Soviet missile and BMD countermeasures that BMD system designs would be expected to overcome. System Requirements. Specification of the missile-interception tasks and sub-tasks that effective BMD systems would have to perform to meet the project threats; specification of passive and active survivability measures for the system. System Designs. Proposals for integrating sensors, weapons, and command and control arrangements into BMD systems that would likely meet system requirements and that could be practicably modified to meet changing threats; and specification of how technologies under research would be incorporated into a BMD system-such a design is called a system architecture. Technology Requirements. Specification of the technologies needed to build the weapon systems required by the overall system design, by the deployment and maintenance plans, and by plans for adaptive evolution of the system to meet changes in the threat; and plan for bringing all technology developments to fruition when needed (full-scale engineering development plan). Manufacturing Requirements. Specification of the materials, manufacturing facilities, tools, and skilled personnel needed to manufacture all system elements. Deployment and Operations Analyses. Proposals for how the designed system can be put into place and maintained (in- 49

57 50 eluding space transportation requirements); and schedules for doing so. Cost Estimates. Estimates for what development, procurement, deployment, and operation of the proposed system design will cost; and proposals for reducing system costs. This chapter will focus on two particular topics: 1. the ways in which system architects for SDIO have related strategic goals to BMD system performance needs, and 2. the general characteristics of the system architectures studied for SDIO. The concluding sections of the chapter will identify areas of analysis within those topics where important work remains to be done. It would be highly unrealistic now to expect system architecture studies to be definitive. Each category of analysis is subject to considerable uncertainty, some of which may never be resolved by analysis and limited experimentation. The architecture analysis will necessarily be tentative and iterative: as new information and ideas emerge, modifications will be inevitable. Moreover, the findings from anal sea in each category will and should affect the findings in other categories. For example, meeting a particular technology requirement may be judged possible, but too expensive. The system architecture design may have to be modified to utilize another technology to carry out the same function. On the other hand, new technological developments may make it cheaper to carry out a function in a way that previous analyses had shown to be too costly. For that reason, the system architects attempt to design evolutionary architectures into which advanced technical developments could be phased as they became available. Even after a commitment had been made to develop a particular technology into a weapon system, the process of full-scale engineering development might prove more difficult than anticipated: alternate systems might have to be designed and developed. Moreover, while it is the goal of the architecture analyses to provide options for meeting a range of potential changes in the offensive missile threat, a fully deployed BMD system might still have to be modified in unanticipated ways if the Soviets were to deploy unforeseen countermeasures. Despite the necessarily tentative nature of system architecture analyses, they compel a coherence in thinking about BMD that would otherwise be missing. They also bring into the open the assumptions implicit in the arguments for and against deploying BMD. Because these analyses will inevitably include assumptions and projections that reasonable people may disagree about, it is important that they be carried out competitively, by more than one group of analysts. Such competition will give both the Administration and Congress a basis for identifying the uncertainties, varying assumptions, and alternative projections of the future that will underlie decisions about BMD. It will also be important, when these analyses are offered in justification of major decisions, that they be independently evaluated. Recognizing the importance of system architecture studies, SDIO late in 1984 awarded contracts to 10 teams of military systems analysis contractors to provide competing analyses at a price of $1 million each. On the basis of that competition, five teams were chosen for $5-million, Phase II architecture studies, which were largely completed in mid In addition, a sixth contractor provided SDIO with analytic support to synthesize the findings of the five competitors into a reference architecture to help guide SDI research. As of this writing, the five Phase II teams had been awarded additional contracts to continue some analytic work common to all and to perform some tasks unique to each. Their reports were due at the end of January It had 1 Three other sets of architecture contracts should also be noted. First, through the Air Force Electronic Systems Division, contracts were awarded to three firms to design battle management and communications systems for a BMD system with land- and space-based elements. This work necessitated definitions of more or less complete BMD system architectures, thus to some extent paralleling the work of the general system architecture contractors, The SDIO has subsequently attempted

58 51 been planned that the five would be narrowed to two competitors in a final phase, but that decision was postponed through Eventually a single contractor team will be chosen to design a BMD system in detail. 2 to better coordinate the parallel work of the battle management systems analyses and the main system architecture studies. Second, the Army Strategic Defense Command awarded three other contracts for study of the battle management and communications systems for BMD composed primarily of groundbased components. Third, late in 1986 SDIO awarded seven contracts to teams composed of U.S. and European firms to begin designs of system architectures for European theater defense against intermediate-, medium-, and short-range ballistic missiles. For the future, SDIO has proposed two new organizations for carrying out work on system architectures. One organization would bean SDI Institute, a federally (and, specifically, SD IO) funded think tank to monitor the work on the actual system architecture to be proposed for deployment by SD IO. The Institute would be independent of particular defense contractors, thus reducing the possibility that the interest of current defense firms in selling hardware to the government would play a role in architecture designs. A second new organization is to be a National Test Bed, which would be a network of computers, communications links, and some sensor hardware for simulating ballistic missile defenses. In some cases, the simulations would be purely conceptual, creating a computer world of BMD systems and offensive systems, and testing various assumptions about each. In other cases, this imaginary world might, with simulated incoming and outgoing data, test computer software actually intended for use in a real BMD system. In yet other cases, actual BMD This report will offer numerous examples from the findings of the system architecture contractors and of SDIO adaptations of such findings. With a few exceptions, we will not cite specific contractor sources for those examples. OTA has not undertaken a systematic analysis and comparison of all the dozens of documents that emerged from the several contractor studies. Therefore, a few selected citations might give an unfair impression of the overall performance of any given contractor. Our purpose here is to convey an understanding of the system architecture analysis process and to report some of the results not to conduct management oversight of any Department of Defense (DoD) contractor. In addition, the system architecture work is continuing, and constant revision of previous findings is both necessary and desirable. Thus any given conclusion might not reflect the current views of the particular contractor. hardware tests might be conducted, with data from the computers being fed into an actual test sensor system, and the sensor system sending processed signals back into the computer simulation. If a full-scale BMD system were deployed, the National Test Bed might then be used for simulated battle exercises of the system. OVERVIEW OF SYSTEMS ARCHITECTURE ANALYSES Initially, each of the system architects undertook the same general task of designing BMD systems whose deployment might begin in the mid-1990s and that might evolve into more advanced systems after the year Each group produced designs that it believed could, when fully deployed, provide nearperfect interception of Soviet ballistic missile reentry vehicles (RVs) forecast for deployment in the mid-1990s.3 Each also argued, however, 3 A mid-1990s threat posed against a BMD system that could not be fully deployed until after the year 2000 is unrealistic. Not all architects used the same threat numbers for the same time frames. The architects did, however, project this baseline threat into larger numbers of reentry vehicles and decoys for later years. They also ran excursions on the baseline threat to explore the impacts of larger and smaller threats on defense effectiveness. The excursions into larger threats, with one exception, do not generally appear in the summary documents produced by the contractors. that lesser percentages of interception would achieve desirable military goals along the lines described in chapter 2 of this report. Goal Specification As part of their analyses, the architects used computerized strategic nuclear exchange models (see next section on this topic) to simulate the numerical results of hypothetical nuclear wars between the United States and the Soviet Union. These simulations assumed various levels of defense capability on the two sides (in general the projected offensive capabilities for the mid-1990s were assumed at this stage) for the purpose of showing what differences those defenses might make.

59 52 From these simulations, the analysts drew conclusions about how defenses might contribute to the goals of security and strategic stability. In chapter 2, we described the kinds of measures used to define BMD effectiveness. In this chapter we will further describe some of the assumptions that went into and conclusions that came out of these strategic exchange simulations. Threat Definition A preliminary step to running the strategic exchange simulations was to state the Soviet offensive threat that BMD systems would be designed to counter. The starting point was an SDIO-supplied projection of the offensive missile forces the Soviets might have in the mid-1990s. From this starting point, the architects made varying excursions, positing possible future Soviet missile developments and deployments. In addition, they hypothesized various types and numbers of anti-satellite weapons that the Soviets might conceivably deploy to attack space-based components of BMD systems. Subsequently, and under different program managers, SDIO began a Red Team program to attempt to anticipate possible Soviet responses to U.S. BMD deployments. A major project of this program has been to bring together groups of experts to attempt to design plausible Soviet countermeasures to the technologies under consideration in other parts of SD IO. These potential countermeasures are then presented to SDIO Blue Teams so that they can adapt their technology research and system designs accordingly. In mid-1987, SDIO presented to the Defense Acquisition Board a proposal to proceed with concept demonstration and validation ( Milestone I ) for the first phase of a Strategic Defense System (BMD system) to be deployed in the mid-1990s. This presentation included an officially approved threat description for that period. In reviewing DoD proposals for any BMD system, Congress should understand whether the officially assumed Soviet threat is responsive i.e., whether it reflects plausible countermeasures that the Soviets could have taken by the time the BMD system were full deployed. System Requirements In showing what numbers of nuclear weapons would have to be intercepted to provide various levels of protection for different types of targets (cf. ch. z), the strategic exchange models also yielded basic requirements for strategic defense system performance. Additional end-to-end computer simulations helped define requirements for interception at each stage of flight. (In SDIO presentations accompanyingmid proposals for an initial, less effective BMD system, this process was reversed. First, a number of warheads to intercept was established, then the strategic goals that might be served analyzed afterward.) Systems Designs The system architecture contractors designed BMD systems intended to intercept a very high percentage of the projected missile threat. The working assumption was that early stages of BMD deployment would be stepping stones to the ultimate goal of protecting cities and people from nuclear ballistic missile attack. The designs were not optimized to less ambitious goals. For example, systems that might protect hardened missile silos but could not serve as elements of city defenses were not considered. Systems designed from the outset to preserve nuclear deterrence might well look materially different from those designed toreplace it altogether. Each architect was asked to design: 1. a system that was both space-based and ground-based; 2 one that was primarily ground-based; and 3. one that was intended primarily for defense of U.S. allies against intermediate and shorter range ballistic missiles. In the second phase of system architecture contracts, analysts placed greatest emphasis

60 53 on the first type of system, somewhat less on the second, and least on the third. Each architect considered systems that might be deployable in the mid-1990s, but each also offered concepts for more advanced systems that might be deployed against more advanced Soviet offensive systems out to the year 2015 or so. For each case, analysts identified countercountermeasures intended to neutralize Soviet attempts to penetrate or directly attack the BMD system. The details of the systems designs (for example, a given type and number of space-based rocket interceptors) were built into simulation models that expanded on the nuclear exchange models described above. These end-to-end simulations represented the details of intercepting ballistic missiles throughout all phases of flight, from rocket boost to warhead reentry. Some of the results of these end-to-end simulations are discussed below. These models also aided tradeoff analyses of various types of BMD system components arranged in various configurations. The models were also used to evaluate excursions in the technological requirements forced by particular types of So viet anti-bmd countermeasures. Technological Requirements The architects quantitatively analyzed the relative costs and effectiveness of various approaches to each defensive task. For example, an analysis might examine trade-offs between highly capable missile-tracking sensors on a few high altitude satellites and less capable sensors on many more low-altitude satellites. Many of these trades are discussed sequent chapters of this report. Operational Requirements in sub- Because system designs are still preliminary, it is difficult to specify their exact operational requirements. The system architects did attempt to estimate the continuing space transportation and maintenance requirements for space-based systems over their lifetime. Other SDI programs are conducting research on the logistics of maintaining various space-based and ground-based systems. costs In general, system architects estimated costs for their nearer-term, interim designs those not including directed-energy weapons for boost-phase missile interception. These systems were estimated to cost on the order of $200 billion, depending on the projected need to respond to various types of Soviet countermeasure. Costs of complementary air defense systems were not included. It should be recognized that, given the conceptual nature of the architectures, accurate cost-estimating is virtually impossible at this stage. It does appear that, with thousands of space platforms envisaged, considerable changes would be needed in the way such equipment is now designed and manufactured if space-based BMD systems were ever to be affordable. In addition, a major new space transportation system would have to be designed, developed, manufactured, and deployed. NUCLEAR FORCE EXCHANGE MODELS: DERIVING REQUIREMENTS FROM GOALS The SDI system architects-and several simulation models assume various U.S. and Soother groups as well-have run several types viet offensive nuclear force levels, beginning of strategic nuclear exchange computer simu- with U.S. Government estimates for lations to try to show how defenses might af- Then they assume various strategic targeting feet the U.S.-Soviet nuclear balance. These plans on the two sides and analyze how the

61 54 attempted execution of those plans might be affected by various levels of defense capability on the two sides. The intermediate measure of defense effectiveness is usually the percentage of nuclear warheads intercepted or its complement, the number of leakers. The models translate the numbers of leakers in various cases into numbers or percentages of different types of targets surviving the attack. (For examples of such target types, see ch. 2, box 2A.) Each type of target, in turn, is given a different weight based on judgments about how U.S. and Soviet leaders might value them. Thus the numbers of different types of targets surviving are translated into surviving strategic value. 4 The percentage of surviving strategic value on the two sides is then linked with particular strategic goals. (For a discussion of goals for BMD and ways of measuring BMD effectiveness, see ch. 2.) In some cases, leakage rates were linked (via asset survival expectations) to strategic goals to show what kind of BMD system performance would be needed given a particular assumed level of offensive threat (for example, see table 3-l). Some Conclusions Drawn From Nuclear Exchange Models Strategic Goals and Defense Leakages The system architects strategic nuclear exchange simulations provide a useful basis for studying BMD performance goals. However, because each architect used a different computer model and different assumptions for the sizes and compositions of future U.S. and Soviet offensive nuclear forces, the results are difficult to compare. With that important qualification, here are some conclusions drawn frequently (but not universally) by the different system architects. First, for a mid-1990s Soviet strategic nuclear 4 In these models the Soviets are assumed to have a larger number of strategic targets than the United States, and the Soviet targets are assumed to be harder to destroy. Part of the difference is due to the existence of numerous nuclear-hardened shelters for Soviet political leaders; see Soviet Military Power, 1987, (Washington, D, C.: Department of Defense) p. 52, threat, a BMD system that allowed a few thousand Soviet RVs to penetrate into the United States might complicate Soviet attack plans, but probably would not stop them from destroying most of their chosen targets. 5 In support of SDIO s mid-1987 proposal for an initial BMD system, other SDIO contractors argued that a strategy of adaptive preferential defense might prevent the Soviets from destroying as high a percentage of certain sets of targets as they would wish (as estimated by U.S. analysts). A system that allowed fewer Soviet RVs to leak through would begin to deny the Soviets certainty of destroying many of the military targets that their planners might have designated. But if the Soviets chose to concentrate on economic targets in the United States, they might still be able to deny the United States the possibility of economic recovery from the nuclear war. (Compare this finding with the second set of projections in table 3-l.) With yet lower leakage, the Soviets could still inflict immense damage on the United States. Note, for example, that 10 percent of an attack with 10,000 nuclear weapons would still result in 1,000 nuclear weapons exploding in the United States. But since the Soviets could not be sure which 1,000 of the 10,000 launched would reach which targets, confidence in achieving precise attack goals on a given set of targets would be low. Analyses also seem to show that if the United States had a relatively highly effective BMD system against a mid-1990s Soviet threat while the Soviets had no BMD, the Soviets would improve their relative strategic situation more by adding defenses to limit damage to themselves than by adding offensive weapons in hopes of increasing the damage they could inflict on the United States. 6 In attempt- s The exact percentages in this conclusion and the others below were apparently classified by the system architecture contractors because the computer simulations from which they were derived include classified estimates of U.S. and Soviet military capabilities. 6 This conclusion assumes that the addition of offenses could not improve the leakage rate the same percentage of every added group of warheads would be intercepted, This is not necessarily a valid assumption: much would depend on the composition of the offensive and defensive forces on the two sides.

62 55 Soviet warheads leaking through Table 3-1. Two Perspectives on BMD Effectiveness and Strategic Goals Expected strategic consequences A. One system architect s strategic exchange model and conclusions Many Increase in Soviet attack planning uncertainties. They are forced to launch all their strategic forces at once or reduce their military objectives. A strategic exchange would result in more losses to Soviets than to the United States. Fewer Extremely few The Soviets could no longer reliably achieve the military goals of a strategic nuclear attack while maintaining a secure reserve of missiles for later attacks. Preserves full range of U.S. strategic offensive force retaliatory flexible response options. Each new Soviet ballistic missile has only a fractional chance of being useful. Survival of a large portion of the population and industrial base, a high proportion of military targets other than strategic offensive forces, and sufficient strategic offensive forces to preseve full range of U.S. retaliatory flexible response options. If Soviets attack only other military targets (not strategic offensive forces), medium-high survival of those assets. Would preserve the full range of U.S. flexible response options in war with the Soviets even if Soviets devoted entire attack to U.S. strategic offensive forces (presumably only if Soviets do not have comparable BMD capability OTA). Assured survival of the Nation as a whole: 3 to 5 /0 U.S. casualties in population attack. Assured survival: Soviet ability to put U.S. population at risk is negligible; the United States needs no strategic nuclear retaliatory capability. Assumptions: Mid-1990s projections of Soviet and U.S. strategic forces. Effectiveness of Soviet BMD not specified. Status of air defenses not specified. Alternate analysis: As U.S. strategic defenses improved, an option for the Soviets would be to change their offensive target priorities to maintain a deterrent assured destruction capability. Instead of concentrating their forces on hardened missile silos, for example, they might concentrate them on key military industries or other economic targets; they might even focus on cities per se. Various non-sdio analysts have previously calculated potential consequences of such nuclear attacks, as indicated below. B. If the Soviets retargeted to maintain assured destruction 10 /0 The Soviets attack industries in the 71 largest U.S. urban areas; the equivalent of 500 l-megaton and 200 to kiloton weapons get through. Of the U.S. population, 35 to 45 percent is killed or injured; 60 to 65 /0 of U.S. industry is destroyed. b 30/0 1 to 2% The Soviets attack industries in the 71 largest U.S. urban areas; the equivalent of 100 l-megaton and 200 to kiloton weapons get through. From prompt blast and radiation effects, 20 to 30 /0 of U.S. population is killed or injured; 25 to 35% of U.S. industry is destroyed. c Case 1: The Soviets attack 77 U.S. oil refineries; the equivalent of 80 l-megaton weapons get through. From prompt blast and radiation effects, 5 million Americans die. The U.S. economy is crippled. d Case 2: The Soviets attack 100 key military-industrial targets with the equivalent of 100 l-megaton weapons. Three million die of blast and radiation effects, another 8 million from fires; dead and injured total 10 to 16 million. e Case 3: The Soviets attack 100 U.S. city centers with the equivalent of 100 l-megaton weapons. Fourteen million die from blast and radiation effects alone, a total of 42 million die from blast, radiation, and fires; total dead and injured are 32 to 51 million. f Assumptions: Total Soviet strategic attack of 10,000 weapons. Air defenses equallv.. effective as BMD. aa@t~d from Martin Marietta Aer~~pace analyses, percentages of weapons leaking and assets SlJrviVl?19 deleted for SeCIJdty ClaSS\fiCatiOrl re&30ns. bfrom us, congre~sj ~conom~c and Social Consequences of ~uclear Attacks On the (/rr/ted Sfates, A Study Prepared for the Joint Committee on Defense Production, Published by the Committee on Banking, Housing, and Urban Affairs, U.S. Senate (Washington, D. C.: US. Government Printing Office, 1979), pp. 4-14, Clbid. df rom us, congress, Office of Technology Assessment, ~tre ~ffects of Nuc/ear War(Washington, D, C.: U. S. Government printing Off ice, May, 1979), pp Calculations on casualties were performed for OTA by the the U.S. Defense Civil Defense Preparedness Agency. About kiloton weapons would have the same blast effects as 60 l-megaton weapons, but the pattern of distribution of blast might in fact do more damage. William Daugherty et al., The Consequences of Limited Nuclear Attacks on the United States, /rrternat/orra/ Security, spring 1966 (vol. 10, No. 4), p 5 F!ndings based on the authors computer simulations. About kiloton weapons have about the same blast effects as 100 l-megaton weapons. Ibid,

63 56 ing to assess the effect on deterrence of various levels of defense, the strategic analysts compared the amount of damage the Soviets might suffer (as a weighted percentage of given types of targets) with the amount the United States might suffer. Differences in surviving (value-weighted) percentages of military targets were assumed to confer strategic advantages or disadvantages that would affect Soviet decisions about how to respond to U.S. weapon deployments, whether to go to war, or whether to escalate a conflict to nuclear exchange. Even very low leakage of the BMD system (and assuming comparable leakage of airbreathing nuclear weapon delivery vehicles) could still kill several million Americans, if that were the Soviet objective. (Note that the alternative projections in table 3-1 suggest higher possible casualties.) This level of protection (given the mid-1990s projected nuclear threat) might assure survival of the United States as a functioning nation but would not assure survival of the whole population. Most of the system architects appeared to believe that in the long run they could design systems capable of keeping out a very high percentage of Soviet ballistic missile RVs (assuming the mid- 1990s projected threat); none appeared to believe that leakage levels compatible with assured survival of the U.S. population would be possible without negotiated limitations of Soviet offensive nuclear forces. U.S.-Soviet Asymmetries With varying degrees of clarity, the system architects use of nuclear exchange models brought out the current-and likely future asymmetries between U.S. and Soviet offensive nuclear forces. The Soviet Union has more ballistic missile RVs than the United States. More of the Soviet RVs are based on land than on submarines, while the reverse is true of the U.S. RVs. The United States has more strategic nuclear bombers and air-and sea-launched cruise missiles than the Soviet Union, while the Soviet Union has a more extensive air defense system than the United States. If the Soviet Union had ballistic missile defenses comparable to those of the United States, the net effect of trying to defend our land-based missiles against a Soviet strike would be to reduce the U.S. ability to carry out planned retaliatory missions. Here is why. If defended, a sizable number of U.S. landbased missiles that might otherwise have been destroyed on the ground might survive a Soviet offensive strike. On the other hand, they would then have to survive defensive attacks as they attempted to carry out their retaliatory missions against Soviet territory. In addition, the U.S. submarine-launched missiles (SLBMs), which would not benefit from the defense of land-based missiles, would also have to face Soviet defenses. Furthermore, if the intercepted SLBMs were aimed in part at Soviet air defense assets, such as radar sites, the ability of U.S. bombers and cruise missiles to carry out their missions might also be impaired. Besides the asymmetries in weapons, there are asymmetries in targets on the two sides. The Soviet Union, for example, reportedly has more than 1,500 hardened bomb shelters for its political leadership. The Soviets also are said to spend copious sums on other types of civil defense. The combination of passive defense measures and BMD might do more to protect valued Soviet targets than BMD alone would to protect valued U.S. targets. Given the asymmetries in U.S. and Soviet weapons and defenses, then, the net effect of mutual deployments of comparable levels of defense could be to weaken, not strengthen deterrence-if deterrence were still measured primarily by the penalty that we could impose on Soviet aggression through nuclear retaliation. (If deterrence were measured by denial to the Soviets of some attack goals other than reducing damage to the Soviet Union, then deterrence might be strengthened.) The United States might compensate for U.S.-Soviet asymmetries in three ways: 1. The United States could attempt to build and maintain BMD that was notably su-

64 57 perior to that of the Soviet Union, so that a greater proportion of the smaller U.S. ballistic missile force could be expected to reach its targets. This was the recommendation of at least one of the SDI system architects, who argued that until very high defense effectiveness levels had been reached, equal defensive capabilities on the two sides might confer an exploitable strategic advantage on the Soviet Union (SDIO officials disagree with this assessment). 2. The United States could attempt to maintain and improve the ability-of its airbreathing weapons (bombers and cruise missiles) to penetrate Soviet air defenses so that the loss in effectiveness of our ballistic missiles was offset by the other means of nuclear delivery. This course was assumed in the calculations of a second system architect. 3. If U.S. strategic defenses against all types of nuclear threat (air-breathing as well as ballistic missile) could be made extremely effective, we might not care about imbalances in punitive abilities on the two sides; the Soviets would have little or nothing to gain by threatening nuclear attack. Then, even a minimally destructive retaliatory ability on the U.S. side should fully deter the Soviets from even contemplating attack. This was the ultimate goal hypothesized by all the system architects. (It should be noted that most, though not all, analysts believe that this kind of deterrence now exists. If so, BMD would not significantly reduce the risk of nuclear war. 7 ) However, some would argue that future Soviet counterforce capabilities, plus Soviet civil defense and perhaps active (BMD and air defense), could reduce prospective Soviet damage to levels acceptable to them. A U.S. BMD system, it is argued would either maintain the That is, given the threat of retaliatory punishment, it would be highly irrational for the Soviets to start a nuclear war. In this view, whatever calculations the Soviets may make about the military effectiveness of their ballistic missiles, the price (in damage to the Soviet Union) would be too high to justify a nuclear attack. survivability of the U.S. deterrent, or equalize the prospective damage on the two sides, or both. In sum, the force exchange models employed by some of the SDI system architects seem to show that BMD performance levels must be high to substantially alter the current U.S.-Soviet strategic nuclear relationship: Some increments of uncertainty could be imposed on Soviet planners by defenses able to intercept about half the Soviet missile force. If an adaptive preferential defense strategy could be executed, significant fractions of some sets of point targets might be protected. The ability to intercept a high percentage of all Soviet strategic nuclear weapons including air-breathing ones (assuming threats projected for the mid-1990s) might actually deny the Soviets the ability to destroy many military targets. However, at such levels of defensive capability, because of asymmetries in U.S. and Soviet strategic postures, U.S. missile and air defenses might have to perform conspicuously better than Soviet defenses to prevent the Soviets from holding an apparent strategic advantage. 8 The design of a system that could, in the long term, protect U.S. cities from potential nuclear destruction seems infeasible without sizable, presumably negotiated, reductions in Soviet offensive forces. At the conclusion of this chapter, we return to the subject of nuclear force exchange models to indicate the scope of future work OTA believes should be carried out if a decision on BMD development and deployment is to be considered fully informed. However, if the United States maintained a substantial bomber-cruise missile threat, if Soviet air defenses were ineffective, and if the Soviets did not pose a substantial bomber-cruise missile threat to the United States, such a Soviet advantage might be avoided.

65 58 Limitations of Nuclear Force Exchange Models Although force exchange analysis is important, applying the results of the analyses requires extreme caution. The greatest danger lies in accepting the numbers generated by the computer as representing reality: they do not. The verisimilitude of a computer simulation can only be checked by comparisons with measured results in the real world that the model is trying to simulate. There has never been and we all hope there will never be a real nuclear war to calibrate the correctness of nuclear force exchange models. Instead, such models combine what is known or estimated about the characteristics of weapons and potential targets on each side with a myriad of personal, even if carefully considered, judgments about how nuclear attacks would take place and what the immediate physical results might be. If national leaders are to make wise use of the outcomes of such analytic models, they need to judge whether they agree with the assumptions that go into the models (see table 3-2). Aside from the many subjective judgments that must go into force exchange models, there are other aspects of the real world that cannot be included in a quantitative computer simulation. The models generally include estimates of prompt casualties from nuclear attacks, but they do not even attempt to account for the longer term medical, social, political, and eco- Table 3-2. Judgmental Assumptions in Nuclear Force Exchange Models Soviet valuation of Soviet targets Estimation of U.S. targets selected by Soviet planners Priorities Soviets would attach to destroying particular targets Soviet estimates of the reliabilities and capabilities of their weapons Soviet estimates of the reliabilities and capabilities of U.S. weapons U.S. estimates of the reliabilities and capabilities of U.S. weapons U.S. estimates of the resistance or vulnerability to nuclear attack of various Soviet targets Estimates of casualties on both sides from nuclear attacks SOURCE: Office of Technology Assessment, nomic consequences of nuclear war. Computer simulations also abstract strategic calculations out of political context. We can only guess, with varying degrees of informed judgment, under what circumstances the Soviets would contemplate starting or risking nuclear war. We do not know how leaders on either side would actually behave in a real nuclear crisis. We do not know, in particular, how and to what degree their decisions would be affected by military planners strategic exchange calculations. In sum, nuclear force exchange models can serve as a useful tool for thinking about the goals we might use BMD to pursue. But they cannot demonstrate as scientific fact that those goals will be accomplished, nor can they offer certainty that the effects of deploying BMD would fulfill predictions. SYSTEM DESIGNS AND END-TO-END MODELS Force exchange models such as those described above can help analysts estimate how many nuclear weapons a BMD system must intercept to achieve various levels of protection. In this way, decisionmakers can set the overall requirements for BMD performance. Much more detailed analysis is needed to evaluate systems designed to meet those requirements. This kind of analysis begins, as do force exchange analyses, with projections of the Soviet missile threat during the period for which one expects to have BMD deployed. In this case, however, analysts must consider more than the destructive capabilities of the offensive missile threat. Analysts must also estimate the precise technical performance of the missiles, the numbers of each type, and the tac-

66 59 tical plans under which the Soviets might launch them. In addition, the analysis has to include possible changes in Soviet offensive forces 9 in response to U.S. BMD deployments. Among the techniques used for this kind of analysis are end-to-end computer simulations, which model both the offensive attack and the roles of each type of BMD component, from the sensor that first detects an enemy missile launch to the last layer of interceptors engaging reentry vehicles as they approach their targets. As table 3-3 indicates, an ICBM flight includes four broad phases: the boost, post-boost, mid-course, and reentry or terminal. System architects for SDI have proposed ways of attacking ballistic missiles in all phases. Space- and Ground-Based Architectures Suggested components and functions of a multi-phase BMD system are outlined in tables 3-4 and 3-5. (Chs. 4 and 5 examine the technology for many of these components in considerable detail.) The SDI system architects subdivided the primarily space-based architectures into nearer- and farther-term BMD sys- Including offensive countermeasures such as decoys and defense suppression measures such as anti-satellite weapons. terns, with the nearer-term systems envisaged as evolving into the farther-term systems as the Soviet missile threat grows and as more advanced BMD technologies become available. Except for the projected timing, the architecture in table 3-4 reflects SDIO s proposal in mid-1987 for a first-phase Strategic Defense System. The design would also be intended to lay the basis for expansion into phase two and three systems. The architectures in table 3-5 draw on information provided by SDIO, but do not constitute their-or anyone else s specific proposal for what the United States should plan to deploy. Instead, the examples provide a framework for analyzing how the parts of a future BMD system would have to fit together to try to meet the requirements set for it. The tables do include the leading candidates for sensors, discrimination, and weapons described by the system architects. The projected dates in the tables reflect OTA rather than SDIO estimates for the earliest plausible periods over which each phase might be deployed if it were proven feasible. The SDI system architects subjected their various BMD constructs to detailed computer simulations. (These are called end-to-end simulations because they attempt to model Table 3-3. Phases of Ballistic Missile Trajectory Phase Duration Description Boost Several 10s to 100s of seconds a Post-boost s of seconds to 10s of minutes b Mid-course About 20 minutes (less for SLBMs) Reentry to 60 seconds Powered flight of the rocket boosters lifting the missile payload into a ballistic trajectory Most ICBMs now have a post-boost vehicle (PBV), an upper guided stage that ejects multiple, independently targetable reentry vehicles (MIRVs) into routes to their targets. If these RVs are to be accompanied by decoys to deceive BMD systems, the PBV will dispense them as well. RVs and decoys continue along a ballistic trajectory, several hundred to 1,000 kilometers up in space, toward their targets. RVs and decoys reenter the Earth s atmosphere; lighter decoys first slow down in the upper atmosphere, then burn up because of friction with the air; RVs protected from burning up in friction with the air by means of an ablative coating; at a preset altitude, their nuclear warheads explode. af.jow in the hundreds of seconds, in the future boost times may be 9reatlY reduced. bpo5t.boo5t dispersal tlme~ may a150 b e shodened, though perhaps with penalties in payload, numbers of mid-course decoys, and accuracy, SOURCE Office of Technology Assessment, 1988.

67 60 Table 3-4. SDIO s Phase One Space- and Ground-Based BMD Architecture Component Number Description Function First phase (approximately ): Battle Management Variable Computers Boost Phase Surveillance and Tracking Satellite Several at high altitude Space-based Interceptor 100s at several 100s of Carrier Satellite km altitudes Probe or Space Surveillance and Tracking System or Space-based Interceptor Carrier Satellites Exe-atmospheric Interceptors (ERIS) 10s 10s SOURCE: Office of Technology Assessment, loos 1000s on ground-based rockets May be carried on sensor platforms, weapon platforms, or separate platforms; groundbased units may be mobile Infrared sensors Each would carry about 10 small chemical rockets or SBIs ; might carry sensors for tracking post-boost vehicles Ground-launched rocket-borne infrared sensors Satellite-borne infrared sensors Satellite-borne infrared sensors Rocket booster, hit-to-kill warhead with infrared seeker Coordinate track data; control defense assets; select strategy; select targets; command firing of weapons Detect ballistic or ASAT missile launches by observing hot rocket plumes; pass information to tracking satellites On command, launch rockets at anti-satellite weapons (attacking BMD system), boosters, possibly PBVs. Acquire RV tracks, pass on to ERIS interceptors Cued by satellite-borne or rocket-borne infrared sensors, home in on and collide with RVs in late mid-course BMD performance from booster launch to final RV interception.) Such simulations help show the interdependence of the system components and the requirements posed for the technologies that go into them. These analyses show that, at least in the long run, intercepting a substantial portion of the missiles in the boost phase and early post-boost phase would be essential to a highly effective BMD system. This conclusion follows from the fact that 1,000 to 2,000 boosters could dispense hundreds of thousands of decoys that would greatly stress mid-course interception. 10 The system architects noted that this boostphase interception task would eventually (barring sizable offensive arms limitations) have to be accomplished by means of directed-ener- 10 SDIO officials point out that an arms control agreement reducing offensive forces would make the defensive job easier and cheaper. On the other hand, the Soviets may not be persuaded to enter into such an agreement unless they can be shown that potential defensive options would make offensive countermeasures on their part futile. gy weapons, rather than by the space based interceptors (SBIs) envisaged for the first stage of BMD deployments. The speed-f-light velocity of directed energy would be needed because the development of faster-burning rocket boosters and faster-dispensing postboost vehicles (PBVs) would eventually permit Soviet missiles to finish their boost phases before the space-based interceptors (SBIs) could reach them. The SDIO contends, however, that interception of PBVs may suffice to meet SDI goals. Although a fast-burn booster would burn out inside the atmosphere, the PBV must clear the atmosphere to dispense light-weiglit decoys. It then would be vulnerable to SBIs. If SBI interception of PBVs were adequate, directedenergy weapons might not be necessary. If successfully developed, though, they might prove more cost-effective. The interplay of offensive and defensive technologies is discussed in more detail in chapters 6, 10, and 11 of this report.

68 1 61 Table 3-5. OTA s Projections of Evolution of Ground- and Space. Based BMD Architecture Component Number Description Function Second phase (approximately ) replace first-phase components and add: Airborne Optical 10s in flight Infrared sensors System (AOS) Ground-based Radars High Endo-atmospheric Interceptors Space Surveillance and Tracking Satellite (SSTS) Space-based Interceptor Carrier Space-based Neutral Particle Beam (NPB) Detector Satellites 10s on mobile platforms 1000s at few 1000s of km. 1000s at 100s of km altitudes 10s to 100s at altitude similar to SSTS 100s around particle beam altitudes X-band imaging radar Rocket with infrared seeker, nonnuclear warhead High-resolution sensors; laser range-finder and/or imaging radar for finer tracking of objects; May carry battle management computers Each carries about 10 small chemical rockets or KKVs. at low altitude; lighter and faster than in phase one Atomic particle accelerator (perturber component of interactive discrimination; additional sensor satellites may be needed) Sensors to measure neutrons gamma rays from objects bombarded by NPB; transmitters send data to SSTS and/or battle management computers Third phase (approximately ), replace second-phase components and add: Ground-based Lasers, 10s of ground-based Several laser beams from each Space-based Mirrors lasers; 10s of relay of several ground sites bounce SOURCE: Office of Technology Assessment, mirrors; 10s to 100s of battle mirrors or off relay mirrors at high altitude, directed to targets by battle mirrors at lower altitudes Track RVs and decoys, pass information to ground battle management computers for launch of ground-based interceptors Cued by AOS, track RVs as they enter atmosphere; discriminate from decoys, pass information to ground battle managers Collide with RVs inside atmosphere, but before RV nuclear detonation could cause ground damage Track launched boosters, postboost vehicles, and ground or space-launched ASATs; Track RVs and decoys, discriminate RVs from decoys; Command firing of weapons On command, launch rockets at anti-satellite weapons (attacking BMD system), boosters, PBVs, and RVs Fire hydrogen atoms at RVs and decoys to stimulate emission of neutrons or gamma rays as discriminator Measure neutrons or gamma rays emitted from RVs: heavier objects emit measurable neutrons or gamma rays, permitting discrimination from decoys Attack boosters and PBVs Battle Management Architecture Specifying Battle Management Architecture Any BMD system architecture will contain a kind of sub-architecture, the battle management architecture. The battle management design shows how BMD system components would be integrated into a single coordinated operating entity. The battle management software, which would direct the battle management computers and control the actions of the system, would carry the burden of integration. A communications system would transmit data and decisions among the battle management computers and between the computers and the sensors and weapons. The system would probably divide the volume in which the battle would be fought into a set of smaller battle spaces. A regional or local battle manager would consist of the bat-

69 62 tle management software and computer with responsibility for controlling the resources used to fight within a particular battle space. The battle manager and the resources it controlled would be known as a battle group. The battle management architecture specifies the following: the physical location of the battle management computers and the nodes of the communications network; the method for partitioning resources into battle groups so that battle management computers have access to and control over appropriate numbers and kinds of sensors and weapons; a hierarchical organization that specifies the authority and responsibility of the battle managers, similar to a military chainof-command; the role of humans in the battle management hierarchy; the method used for coordinating the actions of the battle managers through the battle management hierarchy and across the different battle phases so that handover of responsibility, authority, and resources between boost, post-boost, midcourse, and terminal phases would take place smoothly and efficiently; and the organization of and the method used for routing data and decisions through the communications network, probably organized as a hierarchy that would govern how the nodes of the network were connected. Battle management architectures proposed so far have varied widely in their approach to these issues. For example, some architects proposed placing their space-based battle management computers on the same satellite platforms as the Space Surveillance and Tracking System (SSTS), some on the carrier vehicles, and some on separate battle management platforms; some proposed that the battle managers exchange track information only among neighbor battle managers at the same level of the battle management hierarchy, while others proposed that the same data also be exchanged between upper and lower levels; some architects permitted humans to intervene in the midst of battle to select different battle strategies while others allowed humans only to authorize weapons release. Table 3-6 describes two different battle management architectures that are representative of those proposed. It shows the physical locations of the battle managers, the criteria used for partitioning resources into battle groups, the data exchanged by the battle managers, the methods used for coordinating responsibility and authority between phases of the battle, the degree to which human intervention would be allowed during battle, and the structure of the communications network. Interaction Between Battle Management and System Architecture Battle management architectural decisions would strongly affect the size, complexity, and organization of the battle management soft- Table 3-6. Two Representative Battle Management Architectures Design by location of battle Data exchanged by Method of coordinating Degree of Communications managers Partitioning criterion battle managers between battle phases human intervention network organization Design i: Local battle groups Object tracks Regional battle Humans authorize Two-tiered hierarchy assigned to cover managers control weapons release at SSTS specific Earth-based hand-over between start of battle; can geographic areas phases switch strategies during battle Design ii: Initially geographic, Health (weapon status) All battle managers Humans authorize All nodes in line-ofthen by threat tube information use same criteria for weapons release at sight of each other Carrier (the path along target allocation, start of battle are interconnected vehicles which a group of taking into account missiles travels) locations of other battle managers SOURCE: Office of Technology Assessment, 1988.

70 63 ware. Because of the close relationship between the battle management computers and the communications network, such decisions also would strongly affect the software that controlled the computers forming the nodes of the communications network. A good example of the interaction among system architecture, battle management architecture, and battle management and communications software is represented by the controversy over how widely distributed battle management should be. The two extremes of completely centralized and completely autonomous battle managers and a range of intermediate options are discussed in both the Fletcher and Eastport group reports and considered in all the architectural studies. 11 Physical Organization v. Conceptual Design Analyses often have reflected confusion between the physical organization and the conceptual organization of the battle managers. The physical organization may be centralized by putting all of the battle management software into one large computer system, or be distributed by having battle management computers on every carrier vehicle. Similarly, the software may be designed as: 1. a single, central battle manager that controls the entire battle; 2. a hierarchy of battle managers, with local battle managers each responsible for a small battle space, regional battle managers responsible for coordinating among local battle managers, and a central battle manager coordinating the actions of the regional battle managers; or 3. as a set of completely independent battle managers with no coordination among each other. Any of these three software designs might be implemented using either a centralized or dis- Report of the Study on Eliminating The Threat Posed by Nuclear Balh stic Missiles, Vol. V, Battle Management, Communications, and Data Processing, October This was the only unclassified volume of the Fletcher commission report. See also Eastport Study Group A Report to the Director, Strategic Defense Initiative Organization (Eastport Study Group, Marina Del Rey, CA, 1985). tributed physical organization. Variations on the three designs, e.g., introducing more levels into the battle manager hierarchy, are possible, but infrequently considered. The physical organization and the conceptual design would impose constraints on each other, and factors such as survivability and reliability would drive both. A widely distributed physical design, involving many independent computers, would impose too heavy a synchronization and communications penalty among the physically distributed components of the software to permit use of a centralized conceptual design: the attendant complications in the software would make the battle manager unreliable and slow to react. Physical distribution requires the battle management software on each computer to be relatively autonomous. A system with completely autonomous battle managers would perform less well than a system with communicating battle managers. Accordingly, even a widely distributed physical organization would likely require some communications and synchronization among the battle managers. A centralized physical design might not provide sufficient computer processing power for acceptable performance, but would significantly improve communications among the battle managers. The result might simplify the software development, and lead to greater software reliability. On the other hand, such an organization might result in a poorly survivable system: if the central computer were disabled, the remainder of the system could not function. Integrating Battle Management Architecture With System Architecture Since the system architecture, physical battle management organization, and battle management software design affect each other, all should be considered together. The relationships and interfaces among the battle managers should be defined either prior to or togetherwith definition of the physical organization of the battle managers and their requirements for communication with each other and with sensors and weapons. As the Fletcher report

71 64 stated, The battle management system and its software must be designed as an integral part of the BMD system as a whole, not as an applique. 12 Most of the SDI architectures proposed so far have shown little evidence of an integral design. Software design has been largely ignored, giving way to issues such as the location of the battle management computers and the criteria for forming battle groups. The SDIO has reported that it is attempting to better integrate overall system architecture studies and battle management studies in its current phase of system architecture contracting. However, the system proposed in mid-1987 for demonstration and validation seemed to reflect no such integration. Some Important Results of the System Requirements and Design Work Systems analysis for SDI is still, necessarily, at a preliminary stage. Its most valuable contribution so far has probably been the identification of key issues that research would have to resolve satisfactorily before the Nation could make a rational decision to proceed to development and deployment of BMD. In particular, the analyses have shown the following: Boost-Phase Interception Adequate boost-phase interception of missiles is essential to make the mid-course and terminal interception problems manageable; otherwise, the offense has the opportunity to deploy so many decoys and other penetration aids that they could swamp the other defensive layers. However, an adequate boost-phase interception may, over time, be countered by new of fensive weapons and still have done its job: after deploying all the faster burning boosters and PBVs it could afford to counter the boost-phase defense, the offense may not be able to deploy enough decoys to overwhelm the mid-course defense. 12 Ibid. Ultimate Need for Directed-Energy Weapons As a corollary to the need for effective boostphase interception, it will be important to have a credible long-term system design which includes directed-energy weapons based in space to carry out boost-phase interception against boosters and PBVs that are too fast to be reached by kinetic energy weapons. Without such a credible plan, the boost-phase interceptors would face fairly predictable obsolescence. (It is possible, however, to imagine the development of new SBIs able to penetrate the upper atmosphere; if launched quickly enough, they could then reach some boosters.) Need for Interactive Discrimination Because of the potential for Soviet deployment of hundreds of thousands of decoys that passive sensors may not be able to differentiate from RVs disguised as decoys ( anti-simulation ), mid-course interception is likely to require means of perturbing RVs and decoys and highly capable sensors to detect the differences in the ways the two kinds of objects react. Such means of interactive discrimination have been conceived but not yet built and tested. Interdependence of Defensive Layers Ideally, independent layers of sensors and weapons would carry out interception of each phase of ballistic missile trajectory, thus eliminating common failure modes and common nodes of vulnerability to hostile action. In fact, for practical reasons, the system architects generally produced designs with considerable degrees of interdependence. In addition, as noted above, even if the functions of each layer were performed entirely independently, failure in one phase of interception (the boost-phase, for example) can severely affect the potential performance of succeeding phases. Importance of Integrated Battle Management Architecture Initially, system architecture and battle management architecture studies were separately contracted for, producing large discrepancies among those who had studied each

72 65 Although considerable work on designing BMD battle management remains, analysis so far makes clear the importance of a battle management system that make decisions in a distributed, as opposed to centralized, fashion. Attempting to centralize the decisionmaking would both impose excessive computing, soft- ware engineering, and communications requirements and make the system more vulnerable to enemy disruption. b subject the most. The two sets of studies are apparently now being better integrated, and presumably subsequent designs will reflect that integration. Distributed Battle Management Heavy Space Transportation Requirements The system architecture designs now permit better forecasts of the requirements imposed by space-based systems for space transportation capabilities-capabilities far beyond those the United States now possesses. (Primarily ground-based architectures do not share this problem.) Requirements for Assured Survival There appears to be general agreement on the importance of significantly reducing offensive force developments if one hopes to provide mutual assured survival for the U.S. and Soviet populations. IMPORTANT SYSTEMS ANALYSIS WORK REMAINING The SDI architecture studies have just begun to address the complex problems of designing a working, survivable BMD system with prospects for long-term viability against a responsive Soviet threat. Thus far, the architecture studies have served the useful purpose of helping to identify the most critical technologies needing further development. Future system designers would have to integrate the technologies actually available-and mass producible into deployable and workable weapon systems. Given that the system architects and SDIO are just over 2 to 3 years into an analytic effort that will take many more years, it is not a criticism to say that much work remains. However, it appears to be the case that the analysis supporting the first-phase architecture that SDIO proposed in mid-1987 simply did not address many key questions. The following are further tasks that analysts should carry out to help both the executive and legislative branches judge the potential effects of decisions on BMD. Further Strategic Nuclear Force Exchange Work The strategic nuclear exchange modeling done so far by the SDI system architects provides a useful beginning to the larger and lengthier task of developing the information that will be needed for a national decision on whether to deploy BMD. If the limitations of these kinds of simulations are borne carefully in mind, they can help one to understand how BMD might affect the calculations of U.S. and Soviet national leaders, both indecisions about peace and war and indecisions about long-term strategic policies. They can also help to clarify the assumptions all participants bring to the U.S. national debate about BMD. Introduce Comparability Among Analyses It is desirable to have competing sets of computer simulation models for analyzing the same questions. In that way, decisionmakers could compare differing conclusions and identify the underlying assumptions of each. (Comparisons could also uncover errors in implementation of the models.) Analysts should run different models using the same sets of data about the Soviet missile threat, the same configurations of defensive systems, and the same offensive and defensive strategies and tactics. Thus far, differences in these elements have made the analyses of the system architects difficult to compare and judge.

73 66 Further Analyses of Soviet Offensive Responses The simulations run so far have examined only limited variations on Soviet attack plans in the face of growing U.S. defensive capabilities: the assumption is made that the Soviets have an inflexible list of targets. The Soviets are assumed to optimize their exact attack plan to destroy the highest possible number of those targets at some level of confidence. Suppose, however, that if defenses drastically reduced Soviet confidence in their ability to destroy hardened military targets, they concentrated on softer military and economic targets. Analysts must carry out further exploration of this possibility if decisionmakers are to understand the full implications of BMD for all types of deterrence (see table 3-l). Assumptions About Deterrence An analytic focus on an inflexible Soviet target plan seems to be related to a simplified model of potential Soviet motives for attack. The usual working assumption seems to be that the Soviets would decide to launch a nuclear strike on the United States on the basis of calculations about the probabilities of destroying certain percentages of various types of targets. In this view, above a certain threshold for one or more of these probabilities, the Soviets would be willing to strike, and below it they would not because they could not accomplish their military purposes. One target set would be the weapons and command-andcontrol facilities that would permit a U.S. nuclear retaliation. But the exact role in Soviet decisionmaking attributed to fear of retaliation as opposed to accomplishment of other military objectives remains unclear. The nuclear exchange models should make more explicit their assumptions about the weighings given to denial of military objectives as opposed to the likelihood and intensity of U.S. retaliation as enforcers of deterrence. Analysts should attempt to identify the increment of uncertainty added to the Soviet calculus of nuclear war provided by levels of defensive capability that might increase Soviet uncertainty about achieving attack objectives, but that could not assure denial of those objectives. Many things could go wrong with a nuclear attack precisely scheduled to achieve a specific set of goals (such as knocking out a given percentage of U.S. retaliatory capability). How much uncertainty would a given level of BMD add to that which already exists? What are the potential Soviet responses to this additional uncertainty? 13 To what extent would the increment of uncertainty strengthen deterrence? At what cost per increment of strengthened deterrence? Strategic Stability Analyses Closely related to the question of Soviet attack motivations is the question of strategic stability. In its 1985 report on BMD, OTA emphasized the importance of exploring this question thoroughly. A simplified approach to crisis stability is as follows: in a military confrontation with the United States, Soviet decisionmakers would calculate whether or not they could achieve a given set of military objectives by launching a strategic nuclear first strike. If the objectives seemed attainable, they would strike; if not, they would refrain. The system architects have considered this scenario. Another possibility they should address, however, is that Soviet perceptions of a likely U.S. first strike might affect Soviet behavior. System architects have been understandably reluctant to run or to report extensively on simulations in which the United States is assumed to strike first. Such analyses might imply to some that a change is being contemplated in U.S. policy not to launch a preemptive strategic nuclear first strike. Nevertheless, such analysis needs to be done, not because the United States would launch such an attack, but because the Soviet Union might not believe that it would not. 13 A possibility suggested by one reviewer of the OTA study is that the Soviets discover, unbeknownst to the United States, a way of disabling the U.S. BMD system (perhaps by spoofing its command and control system). Further, the Soviets validate their countermeasure with undetected techniques before actually launching an attack. Certain that their technique will work, and their offensive forces augmented in response to the U.S. defensive deployments, the Soviets in this scenario end up more certain about the probable success of their attack than before,

74 67 It is conceivable, for example, that Soviet strategic exchange calculations could show that a U.S. first strike, backed up by U.S. BMD, might allow the United States to reduce significantly the damage from a Soviet ragged retaliation. 14 On the other hand, a Soviet first strike might have an analogous effect. If the Soviets believed that the United States, expecting a Soviet strike, might strike first, then the Soviets might try to get in the first blow. Thus, they would not make their decision to strike on the basis of accomplishing a clear set of military objectives, but instead on the basis of choosing the less terrible of two catastrophic outcomes. Even if the Soviet Union and the United States avoided a nuclear crisis in which such calculations might play a role, the calculations could still influence the longer-range Soviet responses to U.S. BMD deployments. The Soviets might decide that it was extremely important to them to maintain a credible nuclear threat against the United States, and therefore be willing to spend more on maintaining offensive forces than cost-exchange ratios would seem to justify. Administration officials have repeatedly stated their desire to negotiate (or find unilaterally) a stable transition path to a world in which strategic defenses play a large role. Finding such a path would require careful analysis of the incentives presented to Soviet leaders by U.S. actions. Estimating the consequences of a hypothetical U.S. attack is one key part of such an analysis. Only then might U.S. analysts identify offensive and defensive force levels that both sides could believe served their security. Some of this analytic work has been started, but more is necessary. U.S. Responses to Soviet BMD It is entirely possible that the Soviet Union will not wait until the United States decides whether deploying BMD is a good idea or not, but instead will unilaterally choose to expand 14 A ragged retaliation is one carried out after the first strike has destroyed at least portions of the nation s strategic forces and possibly degraded its command and control system, resulting in a relatively unstructured, diluted counter-attack. its own BMD system. 15 The United States conducts BMD research in part to be able to respond in kind to such a decision. The system architects for SDI have conducted simulations to show how a responding U.S. BMD deployment might restore the U.S.-Soviet strategic balance. Before the United States chose such a response, however, two other kinds of analysis are desirable. First, analysts should compare the BMD option with the option of circumventing Soviet BMD by means of increasing U.S. air-breathing, low-flying cruise missile forces. Second, researchers should determine the ability of U.S. technology to find adequate offensive countermeasures to Soviet BMD. These questions are partly amenable to the strategic exchange modeling technique. In the first case, the model could assume various numbers of cruise missiles with varying levels of probability of penetration in battle scenarios in which Soviet BMD was degrading the ability of U.S. ballistic missiles to get through. Analysts could compare these outcomes to those of similar scenarios in which the U.S. deployed BMD instead of additional cruise missiles. Then they could estimate quantities of BMD and cruise missiles required to produce similar outcomes. This information could provide the basis for cost-effectiveness comparisons between BMD and cruise missiles once data on the actual costs of the two types of systems became available. Similarly, analysts could plug into the simulations the increases in warhead penetration of Soviet defenses caused by U.S. offensive countermeasures. Once estimates were available for the costs of these countermeasures, analyses could develop some idea of the relative cost-effectiveness of offense and defense. lsas ~fitt~ b y the ABM Treaty, the Soviets have retained a limited, nuclear-armed ballistic missile defense system in the Moscow area; they are currently expanding the system to the full 100 interceptors permitted by the treaty, and could conceivably replicate the system elsewhere. They have also constructed a series of phased array radars around the Soviet Union which would provide warning and limited battle management capabilities for such an expanded system.

75 68 Analysis of Alternate Defensive Measures The lesser goals of strategic defense that is, enhancing deterrence by increasing Soviet uncertainty or denial of various military objectives have thus far been considered as preliminary benefits on the way toward extremely high degrees of population protection. Therefore, alternate means of achieving the lesser goals as ends in themselves have not been analyzed. A few examples might clarify this point. Defense of Land-Based ICBMs. If strengthening deterrence by increasing the survivability of U.S. land-based retaliatory forces, especially ICBMs were the goal of deploying BMD, then the system designs done for the SDI might not be optimal. 18 Instead, ground-based, low-altitude interceptors located relatively near the missiles to be defended might be less expensive (unlike cities, hardened missile silos or capsules might withstand low-altitude nuclear explosions). In addition, the United States would want to consider how it could use various forms of mobile or deceptive basing of ICBMsin conjunction with limited BMD to make the enemy s cost of attacking the missiles prohibitive. Careful analysis of the goal of protecting strategic bomber bases from SLBMs launched not far off U.S. shores might also yield different BMD designs combined with different bomber basing tactics. Defense of Command, Control, and Communications Facilities. Similarly the strategic goal of increasing the survivability of the U.S. command and control system for nuclear forces might be achieved by some form of BMD, but the United States should also compare the cost and effectiveness of BMD with those of other measures for making the system more resistant to nuclear attack. Further analysis might show that some combination of passive survivability measures and BMD would be more cost-effective than either alone. Wee U.S. Congress, Office of Technology Assessment, AfX Missile Basing, OTA-ISC-140 (Washington, DC: U.S. Government Printing Office, September 1981). Defense Against Accidental or Terrorist Missile Launches. Protecting the country against 10 or so incoming reentry vehicles is a much different task than protecting it against thousands. While SDI-designed systems might offer such protection as a side-benefit, if this kind of defense were to be the major goal of deploying BMD, one would consider different, much simpler and cheaper architectures than those designed for the SDI. 17 Further System Requirements and Design Work Analyze Additional Threats to BMD System Survivability The SDI system architects recognized that survivability would be a critical feature of any BMD system. They devoted considerable effort and ingenuity to inventing ways to reduce system vulnerability to Soviet attack. The chief threat to survivability they examined was ground-based, direct-ascent anti-satellite weapons rockets that the Soviets could pop up from their territory to attack U.S. spacebased BMD assets with nuclear or non-nuclear warheads. This was a reasonable first approach to the survivability problem: such weapons probably represent the kind of defense suppression weapon most immediately available to the Soviets. If the defense could not counter this threat, then there would be no point in exploring other, more sophisticated threats. In the second round of their horse race competition the system architects did very little analysis of other potential threats to BMD system survivability, particularly longer-term space-based threats. The threat of space mines, satellites designed specifically to shadow and destroy the various space-based BMD components, was not considered in depth. Moreover, no analysis assumed that the Soviets might deploy in space a BMD system For example, a few ground-based, long-range interceptors like the Exe-atmospheric Reentry Interceptor System (ERIS) see ch. 5 could cover the continental United States; existing early-warning radars could give initial track information and a few pop-up infrared sensor probes provide final track information.

76 69 comparable to that of the United States; thus the potential vulnerabilities of such weapon systems to one another were not considered. Instead, it was assumed that the United States would, for the most part, militarily dominate near-earth space. From the statement of work provided to the SDI system architecture contractors late in 1986, it remained unclear whether this assumption would be changed in the follow-on studies to be completed early in Develop Realistic Schedules The system architects were originally instructed to design systems that might enter full-scale engineering development in the early 1990s and be deployed beginning in the m.id- 1990s. The systems they designed would have required challenging technical achievements even under the originally requested SDI budgets. For example, one system architect pointed out that a vigorous technology program did not yet exist for an active space-based sensor crucial to an interim defense intended for deployment in the mid-1990s. Or, to take another example, deployment in the mid-1990s of the space-based systems identified by the architects would require that the United States decide almost immediately to begin acquiring the massive space transportation system that deployment would require. 18 Given the actual levels of SDI funding appropriated by Congress thus far, mid-19% deployment of the kinds of systems initially proposed by the system architects is clearly not feasible. Even with the requested funding, it is unlikely that researchers could overcome all the technological hurdles in time to permit confident full-scale engineering decisions in the early 1990s. Nor is it clear that the full-scale engineering process, including establishment of manufacturing capabilities for the complex systems involved, could be completed in just 3 or 4 years. (For example, the most optimistic expert estimate OTA encountered for engineering full-scale SDI battle management soft- 18The SD1O rque~ $Z!jo million in SUPPleIIIWIt~ funds for fiscal year 1987 to develop technology for low-cost space transportation. ware was 7 years.) In short, the systems designated as interim (similar to those labeled Second Phase in table 3-5) by the system architects would not be likely to reach full operational capability until well after the year Late in 1986, SDIO called on its contractors to orient their work to a much scaled-back system architecture, with scaled-back strategic goals (see the First Phase in table 3-4). Speculations emerged in the press about early deployment options under consideration. Analysis of the phase one designs, however, suggests that even they could not be ready for initial space deployment until at least the mid- 1990s. Nor could they be fully in place much before the end of the century. In the meantime, the Soviet Union might well deploy practical countermeasures against such systems. Specifically, many in the defense community believe that the Soviets could deploy decoys along with their reentry vehicles that would greatly stress the minimal mid-course discrimination capability of a phase-one system. In addition, the Soviets could at least begin to deploy new booster rockets that would drastically reduce the effectiveness of spacebased interceptors (SBIs) in boost-phase defense. Even if the United States could deploy SBIs beginning in the mid-1990s, another question remains: how confident do U.S. decisionmakers wish to be in the long-term viability of BMD before they decide to deploy such systems? Given the state of research on directed-energy devices for BMD, it is highly unlikely that U.S. leaders could have sufficient information by the early 1990s to determine whether full-scale engineering development of phases two and three would be feasible in the following decade. Thus, an early 1990s decision implies a commitment to a space-based BMD whose obsolescence would be made highly probable by the prospect of faster burning Soviet missile boosters, but whose replacement would remain unproven. Develop Credible Cost Estimates The SDIO has properly pointed out that trying to estimate total life-cycle costs for an un-

77 70 precedented system is difficult. The aerospace industry would have to manufacture new components and weapons in new ways. The Nation would need a new space transportation system for a space-based system. The SDIO has agreed to estimate cost goals to indicate the kind of investment that the Nation would have to make in proposed BMD architectures. The system architects were instructed to develop cost estimates in their 1987 studies. Develop Methods for Estimating Cost-Exchange Ratios Between Defense and Offense As this report pointed out in chapter 2, one key criterion for the technical feasibility of the SDI scenario of transition to a defense-dominated world is that there be a favorable costexchange ratio between defense and offense. The system architects did try to address this issue in various ways, but there still seems to be no systematic approach toward it. The problem will be intrinsically difficult, because estimating in advance the costs of the U.S. BMD system will be difficult, estimating the costs of Soviet responses will be more difficult, and predicting Soviet estimates of these quantities will be most difficult of all. Nevertheless, analyses should at least begin to specify what information would permit sufficient confidence that the defense/offense cost-exchange ratio is high enough to justify going ahead. The system architecture contractor teams were instructed to address the problem in their 1987 work. Assess the Role and Costs of Complementary Air Defenses The Strategic Defense Initiative Organization is specifically limited to defense against ballistic missiles. The Air Force has undertaken an Air Defense Initiative, though at funding levels far below that of the SDI. Nevertheless, at least at the systems analysis level, U.S. decisionmakers need an integrated understanding of the role that air defense would have to play if ballistic missile defense were to achieve such goals as increasing Soviet uncertainty about attack success, denying Soviet abilities to destroy high percentages of certain types of targets, or protecting the population from nuclear attack. Moreover, insofar as BMD requires air defense to accomplish its purposes, the feasibility and affordability of air defense against possible Soviet attempts to circumvent BMD need to be included in any ultimate analysis of the feasibility of BMD.

78 Chapter 4 Status and Prospects of Ballistic Missile Defense Sensor Technology

79 CONTENTS Page Introduction Sensors Proposed SDI Sensor Systems Sensor Technology Sensor Technology Conclusions Phase Phase Phase Box Box Page 4-A. Sensor Resolution Limits Figures Figure No. Page 4-i. Major SDI Sensors and Weapons Relations Between Temperature and Electromagnetic Radiation Scanning Pattern for Satellite Sensor Illustration of Three Techniques for Estimating the Three- Dimensional Position of a Target in Space Spectral Response of Two Objects at Different Temperatures a Diffraction-Limited Range for Ten-Meter Resolution b Range Limitedly Number of Detectors for Ten-Meter Resolution Mirror Size Plotted v. the Operating Wavelength of a Sensor System Illustration of an Imaging Radar Viewing a Spinning Conical Target Tables Table No. Page 4-1. Summary of Typical Sensor Requirements Key Issues for Passive Sensors Key Issues for Interactive Sensors

80 Chapter 4 Status and Prospects of Ballistic Missile Defense Sensor Technology INTRODUCTION Much of the public debate on ballistic missile defense (BMD) technologies centers on futuristic weapon systems such as lasers, rail guns, and particle beams. The Strategic Defense Initiative Organization s (SDIO) initial BMD system design, however, does not include any of these exotic weapons. 1 Rather, it calls for space-based interceptors (SBI) to collide with Soviet intercontinental ballistic missile (ICBM) boosters and post-boost vehicles (PBVs), and for high acceleration ground-based missiles to destroy Soviet reentry vehicles (RVs) by direct impact. The sensor systems required to detect, identify, and track up to several hundred thousand targets may be more challenging than the actual kinetic energy weapons: it may be more difficult to track targets than to destroy them, once tracked. The technical feasibility of a first-phase deployment, then, may depend primarily on major technical advances in the areas of sensors and chemically propelled rockets, and less on the availability of rail-gun or laser weapons systems. Accordingly, this report emphasizes these more conventional technologies. Nonetheless, the more exotic weapons technologies could become important in second-or *Some BMD architecture contractors did, however, call for rather exotic beam sources for interactive discrimination, in which targets would be exposed to sub-lethal doses of particle beams or laser beams and their reactions measured to distinguish between reentry vehicles and decoys. See section on interactive discrimination. Recently, SDIO officials have spoken of entry level directedenergy weapons that might constitute part of second-phase BMD deployments. The utility of such weapons would depend on the pace and scope of Soviet countermeasures. Note: Complete definitions of acronyms and initialisms are listed in Appendix B of this report. third-phase BMD systems deployed in response to Soviet countermeasures. For example, if the Soviet Union deployed fast-burn boosters that burned out and deployed their RVs (and decoys) before they could be attacked by slow-moving chemically-propelled rockets, then laser weapons might be essential to attack ICBMs in their boost phase. These directed-energy weapons (DEW) would require even more accurate sensors, since their beams would have to be directed with great precision. Thus, the required sensor technology improvements might continue to be at least as stressing as weapons technology requirements. Some of the major sensor and weapon components proposed by Strategic Defense Initiative (SDI) system architects for both near- and far-term deployments are listed in figure 4-1 (also see ch. 3). This chapter describes sensors; weapons, power systems, communications systems, and space transportation required to implement a global BMD system are described in chapter 5. For each technology, chapters 4 and 5 discuss: the type of system suggested by SDI architects, the technical requirements, the basic operating principles, the current status, and the key issues for each technology. The systems aspects of an integrated BMD system are discussed in chapter 6. Computing technologies are discussed in chapter 8. Technologies for offensive countermeasures and counter-countermeasures are deferred until chapters 10 through 12 (as of this writing, available only in the classified version of this report). 73

81 74 Figure 4-1 Major SDI Sensors and Weapons NPB detector SDI sensor systems: BSTS-Boost Surveillance and Tracking System (infrared sensors) SSTS-Space Surveillance and Tracking System (infrared, visible, and possibly radar or laser radar sensors) AOS-Airborne Optical System (infrared and laser sensors) TIR-Terminal Imaging Radar (phased array radar) NPB-Neutral Particle Beam (interactive discrimination to distinguish reentry vehicles (RV s) from decoys; includes separate neutron detector satellite) SDI weapons systems: SBI-Space-Based Interceptors or Kinetic Kill Vehicles (rocket-propelled hit to kill projectiles) SBHEL-Space-Based High Energy Laser (chemically pumped laser) GBFEL-Ground-Based Free Electron Laser (with space-based relay mirrors) NPB-Neutral Particle Beam weapon ERIS-Exoatmospheric Reentry vehicle Interceptor System (ground-based rockets) HEDI-High Endoatmospheric Defense Interceptor (ground-based rockets)

82 75 SENSORS Sensors are the eyes of a weapons system. In the past the human eye and brain have constituted the primary military sensor system. A soldier on the battlefield would: look over the battlefield for possible enemy action (surveillance); note any significant object or motion (acquisition); determine if the object was a legitimate target (discrimination); follow the enemy motion (tracking); Aim his rifle (weapon direction), fire; look to see if he had killed the target (kill assessment); and if not, reacquire the target (retargeting), aim, and shoot again. Ballistic missile defense entails these same functions of target surveillance, acquisition, discrimin ation, tracking, weapon direction, kill assessment, and retargeting. BMD sensors, however, must have capabilities of resolution, range, spectral response, speed, and data storage and manipulation far beyond those of the human eye-brain system. Proposed SDI Sensor Systems The following sections describe five representative sensor systems. Most of the five SDI system architecture contractors (see ch. 3) recommended some variation of these sensor systems. The primary attack phase and recommended sensor platforms for each type are summarized in tables 1-1 and 1-2. Boost Surveillance and Tracking System (BSTS) The BSTS would have to detect any missile launch, give warning, and begin to establish track files for the individual rockets. Most system architects proposed a constellation of several satellites in high orbit. Typical BSTS characteristics are summarized in the classified version of this report. Each BSTS would carry a sensor suite that would monitor infrared (IR) emissions from the Figure Relatlons Between Temperature and Electromagnetic Radiation Target radiation bands Temperature scales (Peak radiation wavelength) 1 1 I 6,000 Kelvin ( O K) rocket plumes (see figure 4-2). From their very high altitude, these sensors would have relatively poor optical resolution. Track files could be started, but the Space Surveillance and Tracking System (SSTS) or other sensors at lower altitude might be required to achieve the track file accuracy needed for some BMD functions. 2 2 Space-based interceptors (SBIs), formerly called space-based kinetic kill vehicles (SBKKV), which have their own horning sensors, could operate with the resolution given by a BSTS sensor.

83

84 77 would supply adequate coverage around the world for submarine-launched missiles. 3 Redundancy would be necessary for survivability and for stereo viewing of the targets. These SSTS satellites might be essential for much of the mid-course battle, so some SSTSS must survive at most locations. 4 The SSTS satellites would carry one or more long-wave infrared (LWIR) sensors for tracking the somewhat warm PBVs and cold RVs. These LWIR sensors could not detect RVs by looking straight down against the relatively warm earth background. Rather, they would look only above the horizon, in a conical or coolie hat pattern which would afford the necessary cold space background for the IR detectors. Thus each SSTS would monitor targets that were far from the satellite. Those targets closest to each SSTS would pass below its sensors, undetected; they would have to be observed by more distant SSTS satellites (see figure 4-3). This problem could be alleviated if sensing at other wavelengths, e.g., in the visible range, were to be feasible. For some missions, such as cueing DEW sensors, the SSTS might include short-wave infrared (SWIR) and medium-wave infrared (MWIR) sensors to track booster exhaust plumes. This would duplicate to some extent the BSTS function, but with much better resolution. 5 These sensors might have limited fields of view, so that each SSTS platform would require several IR sensors to cover all the threats. These SWIR/MWIR sensors could look down against the Earth background, since they would be monitoring the hot plumes. Several architects recommended placing laser systems (and some suggested microwave radars) on the SSTS. Lasers might be needed More recent SDI studies have recommended fewer satellites. 4Akematively, pop-up IR probes on ground-based rockets could observe the midcourse battle. These probes would have to be based at high latitudes to get close enough to observe the beginning of mid-course missile flight. Otherwise, they could be based in the northern United States to view the late midcourse. 5 An SSTS could not achieve the pointing accuracy needed by DEW satellites; each DEW platform would have to carry its own high-resolution optical sensor. An SSTS constellation might aid the battle manager in designating targets for DEWS. Figure Scanning Pattern for Satellite Sensor A SSTS Above-the-horizon LWIR conical scan pattern "Collie hat above the horizon scan pattern for the LWIR sensors on the SSTS which could only detect the cold RV's against the cold background of space. The targets labeled A could be detected by this SSTS platform, whereas the closer targets labeled B could not be detected against the warm earth background. These B targets would have to be tracked by another, more distant SSTS satellite. to designate or illuminate targets for homing space-based interceptors (SBIs). Laser radar (Ladar) systems might be required for all of the interactive discrimination systems, just to determine the target s position with sufficient accuracy. This would be particularly true for tracking cold RVs, which could be passively detected mainly by LWIR sensors with inherently poor resolution, 6 or for discriminating and designating an RV in the presence of closely spaced objects (that often are decoys). In any case, a laser radar could supply the range to the target, which is necessary to generate three dimensional track files from a single platform. The resolution angle of a sensor is directly proportional to wavelength; long wavelengths such as LWIR produce large rese lution spots in the sensor focal plane, or large uncertainty in the target s location. Therefore shorter wavelength laser radars may be needed to accurately measure target position.

85 78 The SSTS might also carry some battle management computers, since the SSTSS would be above the battle and to some extent less vulnerable than lower altitude weapons platforms, and because they would generate most of the track-file information essential for assigning targets to weapon platforms. The SSTS originally conceived by the system architects for ballistic missile defense now appear too complicated, too expensive, and possibly too far beyond the state of the art of sensor technology for deployment in this century. As a result, there was some discussion in late 1986 and early 1987 of launching early SBIs without any SSTS sensor, placing minimal sensor capability on each SBI carrier vehicle instead. There would probably be no sensor capability enabling SBIs to kill RVs in mid-course. The phase-one architecture submitted to the Defense Acquisition Board in June and July of 1987 was vague about mid-course sensors: there was a Midcourse Sensor (MCS) program, but no system concept. The MCS might consist of SSTS sensors, or ground-based surveillance and tracking (GSTS) rockets or probes, or SWIR/MWIR (or other) sensors on some of the kill vehicle carrier satellites. These sensors would apparently locate targets for the ground-based exe-atmospheric reentry vehicle interceptor system (ERIS) interceptors. More recently, an MCS study proposed a combination of the three sub-systems. The SDIO ended development work on the original SSTS program and let new contracts in mid-1987 to design a less complex SSTS system. The classified version of this report contains the range of parameters specified by the original, more comprehensive system architectures. The new designs could not by themselves furnish precise enough data to direct SBIS to RV targets. Airborne Optical Adjunct (AOA) The AOA would test technology for a new sensor addition to terminal defensive systems. The SAFEGUARD BMD system, operated in partial form in the 1970s, relied exclusively on large, phased-array radars to track incoming warheads. There were no optical detectors, The resolution and range of these ground-based radars was adequate (assuming they survived) to direct nuclear-tipped Spartan and Sprint missiles to the general vicinity of target RVs. Such radars would not be adequate as the only guidance for the non-nuclear, hit-to-kill vehicles proposed for SDI: these interceptors would require on-board homing guidance systems. The AOA would test LWIR technology similar to that in the SSTS program, but deploy it on an aircraft flying over the northern United States. The sensor system has been designed and is being fabricated. Above most of the atmosphere, this sensor could look up against the cold space background and track RVs as they flew through mid-course. Resolution would be relatively coarse: a follow-up system based on this technology might eventually be able to direct ground-based radars, which in turn would hand target track data over to high speed hit-to-kill projectiles. These projectiles would derive their final target position from on-board homing sensors. The AOA aircraft might also include laser range-finder systems to supply accurate estimates of the distance to each target-and possibly to discriminate Photo credit: Strategic Defense Initiative Organization Airborne Optical Adjunct (AOA) In a strategic defense system, airborne sensors might be used to help identify and track targets and to guide ground-based interceptors to them. The AOA will validate the technology to acquire targets optically at long ranges, and to track, discriminate and hand data over to a ground-based radar. It will also provide a data base that would support future development of airborne optical systems. Sensors have been fabricated and tested and test flights will take place soon. The model shows the sensor compartment on top and the crew stations in the interior of the aircraft.

86 79 decoys from RVs by measuring minute velocity changes caused by drag in the upper atmosphere. System architecture contractors proposed tens of AOA-like aircraft as part of a sensor system. Some proposed rocket-borne, pop-up probes with LWIR sensors for rapid response in a surprise attack until the aircraft could reach altitude. There is some uncertainty regarding the infrared background that an airborne sensor such as AOA would see. Sunlight scattered from either natural or (particularly) man-made noctilucent clouds might obscure the real RV targets. These clouds form at altitudes from 60 to 100 kilometers (km). During a battle, the particles ablating from debris reentering the atmosphere would form nucleation centers. Long-lived ice crystals would grow at these centers, possibly creating a noisy infrared background that would obscure the real targets arriving later. Intentional seeding of these clouds is also a possibility. 7 Ground-Based Radar (GBR) Large phased-array, ground-based X-band (8-12 GHz frequency) radars might work in conjunction with optical sensors to track and discriminate incoming warheads from decoys. These radars could receive target track data from those sensors and then use doppler processing to create a pseudo-image of the warheads by virtue of their spinning motion. Nonrotating decoys or decoys with different shapes or rotation rates would produce different radar signatures. Ground-based radars would also measure the effects of the atmosphere, identifying light decoys that would slow down more than the heavy RVs. These radars might guide or cue the endoatmospheric HEDI and FLAGE-like interceptor rockets and the ERIS exoatmospheric interceptors (see ch. 5). See M.T. Sandford, II, A Review of Mesospheric Cloud Physics, Report No. LA (Los Alamos, NM: Los Alamos National Laboratory, October 1986.) The GBR concept very recently supplanted the proposed Terminal Imaging Radar (TIR) system in SDIO planning. The latter would have had a much shorter range (thereby not being useful for cueing the ERIS interceptor) and much less resistance to anti-radar countermeasures, such as jamming. Some radar concepts call for deployment on railroad cars to evade enemy attack. Neutral Particle Beam (NPB) Interactive Discrimination While several interactive discrimination techniques have been proposed (see section be low on interactive discrimination), the NPB approach has thus far received the most attention and development funds. A series of full space-based tests was planned for the early 1990s, but has been subjected to budgetary cutbacks. A 50-MeV 8 NPB source was to be placed in orbit along with a sensor satellite and a target satellite to measure beam characteristics and to begin interactive tests. The primary detection method would be to monitor the neutrons emmitted by the target after irradiation by the NPB, although gamma rays, x-rays, and ultraviolet radiation might also be useful for indicating whether targets had been hit by the neutral particle beam. The NPB accelerator might be located 1,000 km from the target. The neutron detectors might ride on separate detector satellites closer to targets, although they could be collocated on the NPB platform under some circumstances. A single NPB discrimination accelerator system might weigh 50,000 to 100,000 kilograms (kg), making it the heaviest element proposed for a second-phase BMD. 9 Over 100 NPB satellites and several hundred neutron detector The energy of a beam of particles is measured in electron volts or ev, the energy that one electron would acquire traveling through an electric field with a potential of one volt. The energy of beam weapon particles would be so high that it is measured in rniuions of electron volts, or MeV. One MeV is equal to 1.6x1O 3 joules; each particle carries this amount of energy. A far-term, robust BMD system might also include very heavy directed-energy weapons.

87 80 platforms might be required for a global discrimination system. 10 Sensor System Requirements Technical requirements for BMD sensors are discussed below for each sensor function: surveillance, target acquisition, identification, tracking, and kill assessment. Surveillance and Target Acquisition Requirements A surveillance and target acquisition system would have to detect the launch of any missile, either ground-based or submarine-based, and render accurate positional information to the BMD weapon system. Some SDI weapon systems would require very high resolution sensors. A laser beam, for example, would have to be focused down to a spot as small as 20 to 30 cm in diameter to produce the lethal intensity levels for projected hardened missiles. 11 A DEW sensor must therefore determine the missile location to within a few tens of cm so as to keep the laser focused on one spot on the target. As an illustration of what is practical or impractical, note that if the sensor were placed in geosynchronous orbit at 36,000 km, just a few sensor satellites could survey the entire earth. But at this high altitude the sensor s angular resolution would have to be better than 8 nanoradians, or one part in 125,000, Between 100 to 200 flights of the proposed Advanced Launch System (ALS) might be required to lift a full constellation of 100 NPB discibliil ators into space. For a discussion of the number of elements in a useful NPB system, see American Physical Society, Su ence and Technology of Directed Energy Weapons: Report of the American Physical Society Study Group, April 1987, pp. 152 and I For e~~ple, a 90 Mw laser operating at one micrometer (pm) wavelength would require a mirror as large as 10 m in diameter to achieve the very high brightness IOX W/sr) required to destroy hardened (i.e., able to resist 20 KJ/cm2) targets. A 10 m mirror would would project a 20-cm diameter spot at 2,000 km or 40 cm at 4,000 km, which are typical ranges for the proposed directed energy platforms. See chapter 5 on directed energy weapons for more details. One radian is equal to 57.3 degrees; one nanoradian is IxIO - radian or one billionth of a radian. This high resolution is clearly beyond the realm of practical sensor systems. 13 Resolution improves directly with reduced distance to the target. Therefore a reasonable alternative-one being examined-would be to place many sensor satellites at lower altitudes. Even a constellation of sensor satellites at altitudes around 4,000 km would not be adequate for directed energy weapons: positional uncertainties for sensor satellites combined with vibration and jitter would preclude the transmission of target positions to weapon platforms with 10-cm accuracy. Therefore each DEW satellite would need its own sensor to provide the final pointing accuracy. Sensor satellites might supply broad target coordinates to each weapon platform. Homing kinetic energy weapons (KEW) would require less accurate information from a remote sensor: a homing sensor on an SBI itself would give the fine resolution needed in the last few seconds to approach and collide with the target. Still, the SBI must be fired toward a small volume in space where the intercept would occur several hundred seconds after it had been fired. The sensor system must locate each target in three dimensions. Target Identification or Discrimination Requirements Ballistic missile defense (BMD) sensors would not only have to detect missile launches, but they would also have to identify targets. Identification requirements would vary considerably during missile flight. During the boost phase, a sensor would first distinguish between missile exhaust plumes and other natural or man-made sources of concentrated heat. Given adequate spatial resolution, a smart sensor with memory could separate moving missiles from stationary ground-based sources of heat. The location of the missile launcher and the missile s dynamic characteristics (acceleration and burn time for each stage, pitch ma- ISFor exmple, even ~ titraviolet sensor, which would have the best resolution due to its short wavelength, would require a 45-m diameter mirror to achieve 8 nanoradian resolution.

88 81 neuvers, stage separation timing, etc.) should permit identification of missile type and probable mission. Eventually a low altitude sensor would have to identify the booster body (as opposed to the hot plume), either by geometric extrapolation or by generating an IR image of the booster tank. 14 The post-boost phase is more complicated. Most missiles carry a PBV or bus which may include 10 or more individual warheads in RVs. These RVs are individually aimed at separate targets: the PBV maneuvers and mechanically ejects each RV, one at a time, along a different trajectory. A BMD sensor system might detect heat from a PBV propulsion system as it made these multiple maneuvers. However, PBV propulsion energy is far less than main booster engine energy, making tracking (at least in the SWIR/MWIR range) more difficult in the post-boost phase. Once ejected, cold RVs would be even more difficult to detect and track. 15 This reduced signal level could be partially offset by arranging the sensor satellite to view its targets against the cold space background instead of the warm and noisy Earth background, as in the boost phase. The sensors would have to look above the horizon, generally limiting detection to distant targets over the Earth s limb. Since detection becomes more difficult at longer ranges, this above-thehorizon (ATH) detection of cold RVS would be more difficult than sensing very hot booster plumes against the earth background. If the United States deployed a BMD system, Soviet missiles would probably disperse decoys along with nuclear-armed RVs. Decoys might be simple, aluminum-covered balloons weighing 1 kg or less, or they might be somewhat more sophisticated decoys shaped like 14A boos~r body, at 3000 K is cold compared to its hot Plume, but it is still warmer than the cool upper atmosphere at about 2200 K. An LWIR sensor could therefore image the booster body against the Earth background at fairly long ranges, using wavelengths which were absorbed by the upper atmosphere. ICBM boosters typically radiate millions of watts per steradian (W/sr), PBVS hundreds of W/sr, and RVS a few W/sr. (A steradian is the measure of a solid angle, defined as the ratio of the surface area subtended by a cone divided by the square of the apex of that cone.) an RV with similar infrared and radar signatures. Simple decoys might be tethered to an RV within a few tens of meters: defensive sensors would then require higher resolution to separate decoys and RVs. Alternately, an RV could be placed inside a large balloon, a technique known as anti-simulation : the RV is made to look like a decoy. The most sophisticated decoys, called thrusted replicas (TREPs) might even have propulsion so they could push into the atmosphere during reentry to simulate the heavy RV s reentry characteristics. The total postboost and mid-course threat cloud could contain something like 10,000 RVs, hundreds of thousands of decoys, and thousands of burntout rocket stages and PBVs, all traveling through space at 7 km/s. In the same trajectories might be literally millions of fragments from boosters destroyed by SBIs in the boost and post-boost phases. 16 In principle, a BMD weapons system could fire at all of these objects, but the costs would be prohibitive. Therefore the sensors for a second- or third-phase BMD system with midcourse capability would have to discriminate effectively between RVs and the many decoys and debris. In the post-boost phase, there would be some basis for discrimination. A sensor could, in theory, monitor PBV motion during deployment of RVs and decoys. Decoys would produce less PBV motion than the heavier RVs as they were ejected from the PBV. This distinctive motion might be detected, assuming that the Soviets did not cover the PBV with a shroud to conceal the dispersal of decoys, or that they did not appropriately alter the thrust of the PBV as its RVs dispersed. In the mid-course phase, discrimination would become even more difficult. All the objects would travel together in a ballistic, freefall flight. Light decoys would not be slowed down by atmospheric friction until they descended to the km altitude range the same altitude range that constrains deploy- %ee chapter 10 for details on countermeasures to BMD.

89 82 Photo credit: U.S. Department of Defense COBRA JUDY Radar A new radar had been developed and installed on the COBRA JUDY ship. This improves the capability of the U.S. for making measurements on reentry vehicles in flight. ment of rising decoys in the post-boost phase. If decoys had the same signatures or characteristics of RVs as seen by conventional infrared and radar detectors, then conventional discrimination of RVs from decoys would become extremely difficult. Mid-course discrimination is one of the most crucial challenges facing the SDI technology development program. The BMD sensors would also have to detect and track defense suppression threats such as direct-ascent anti-satellite (DAASAT) missiles or space-based ASATs which might attack BMD defensive assets in space. The sensors should therefore keep track of all of the BMD weapons platforms in a given battle space, allowing the battle manager to determine which objects were likely targets and which weapons should engage the threat. Target Tracking Requirements Passive IR sensors on a single BSTS or SSTS satellite could only measure the target position in two angular coordinates. Each target must be located in three dimensions to allow the battle management computer to calculate the expected collision point of weapon and target. Three techniques could furnish three dimensional data: stereo imaging, ranging, or ballistic trajectory prediction (see figure 4-4). Two or more separated sensor satellites could generate stereo data. This would require a computer to correlate data from multiple sensors and could become very complicated with 40 or 50 sensors generating data from thousands or hundreds of thousands of targets. Alternatively, a laser range-finder and a passive IR two-dimensional imager together on one satellite could generate three dimensional information. A laser range-finder would determine the distance to the target. With a direct, one-to-one correlation between two target angles from a passive sensor and a third range coordinate from a laser, computational requirements would be reduced by eliminating the need to correlate data from separate platforms. Finally, for objects traveling in space on a ballistic, free-fall trajectory, Kepler s equations

90 83 Figure Illustration of Three Techniques for Estimating the Three-Dimensional Position of a Target in Space Stereo viewing IR angle/angle plus range finder #1 #2 IR sensor B IR sensor A Ballistic trajectory estimation (from one passive sensor) Time T-3 Time T-2 Tme T-1 I #2. IR sensor In the first view Sensor A could not distinguish between Target # 1 and Target #2. Stereo viewing from two or more separate satellites with passive IR sensors eliminates this ambiguity. Relatively complicated software is required to correlate data from each sensor. The other two techniques can predict three dimensional information from one platform, eliminating the requirement for multiple satellite sensor data correlation; a laser range finder determines the range or distance to a target by measuring the travel time for a pulse of light from the platform to the target and back, uniquely determining position with one measurement. The ballistic trajectory prediction approach uses only the passive IR sensor, but requires three or more measurements at different times to compute the target s path through space.

91 84 of motion may be applied: a passive sensor could determine the path of an RV in three dimensions by measuring its two-dimensional position three or more times. This trajectory prediction approach requires more time (hundreds of seconds) to build up an accurate track: this would be adequate for the mid-course phase. It would require more data storage and processing than the laser range-finder technique, but only one passive sensor. Kill Assessment Requirements Sensors would also have to determine whether a missile or RV had been disabled or destroyed. Missed targets would have to be retargeted, and disabled targets should be ignored throughout the remainder of the battle. Kill assessment should be straightforward for most KEW projectiles, since their impact would smash targets into thousands of pieces. However, some SBIs might partially damage a booster by clipping anon-critical edge, leaving the bulk of the missile intact. In this case the sensor might judge a missile killed if it veered sufficiently off-course to anon-threatening trajectory. Damage to targets attacked by laser or particle beam weapons might be more difficult to diagnose. A laser beam might conceivably burn through a critical component without detectable damage, yet divert a missile from its intended course. More likely, the laser would disintegrate the missile body, which is highly stressed during acceleration as demonstrated by a ground-based high-energy laser test at the White Sands Missile Range. 17 Damage due to particle beams or electron beams might be more difficult to detect. Neutral particle beams, for example, might penetrate several cm into a missile or RV, destroy- 17 The mid-range infrared advanced chemical laser (MIRACL) at White Sands Missile Range in New Mexico was aimed at a strapped-down Titan missile second stage. The missile was mechanically loaded with 60 psi of nitrogen gas to simulate the 4-g load and propellant conditions that it would experience in an actual flight. After approximately 2 seconds of exposure to the laser beam, which had a power greater than 1 megawatt, the Titan booster completely ruptured, shattering into fragments as heating of a roughly 1 m 2 area destroyed the mechanical integrity of the booster skin. ing critical electronic components without any apparent external damage. An RV might be effectively killed with respect to its mission at much lower particle beam energy than that necessary to show detectable damage. On the other hand, NPB system designers could increase particle beam fluence to levels that would assure electronics destruction (say 50 joules/gram (J/g) only 10 J/g destroys most electronics) as long as the target were hit. Kill assessment would then become hit assessment : if the beam dwelled on the target long enough to impart 50 J/g, then the electronics could be judged killed. With this approach, NPB weapons would be effectively lethal at lower energy levels than that needed for melting aluminum or causing structural weakness (500 to 1,000 J/g). Relying on this indirect kill assessment would require confidence that the Soviets had not shielded critical internal electronic components from NPB radiation. Table 4-1. Summary of Typical Sensor Requirements Surveillance: Coverage Global Targets ICBM s, SLBM s, direct ascent ASAT s, space mines, and one s own BMD assets, including all sensor and weapons satellites and launched SBIs Target Discrimination: Boost Phase ICBM/SLBM/DANASAT Post-boost & midcourse PBV, RV, light decoy, replica, thrusted replica, & debris Terminal RV & thrusted replica Tracking: Targets ICBM s 1,400-2,000 SLBM s 1,000-1,500 DANASAT s: 1,000-16,000 PBV s 2,400-3,000 RV s 8,000-15,000 Decoys hundreds of thousands Track file position, velocity, & acceleration in 3-D Kill assessment: KEW destruction Laser destruction NPB hit assessment or other SOURCE: Office of Technology Assessment, 1988.

92 85 Sensor Technology Three types of sensors might satisfy portions of these BMD requirements: passive, active, and interactive. Passive sensors rely on natural radiation emitted by or reflected from the target. Active sensors, such as radars, illuminate the target with radiation and detect the reflected signal. Interactive sensors (a term unique to the SDI) would use a strong beam of energy or cloud of dust-like particles to perturb targets in some measurable way (without necessarily disabling it) so that RVs could be discriminated from decoys. For example, the cloud might slow down light decoys much more than heavy RVs, or penetrating particle beams might create a burst of neutrons or gamma rays from RVs but not from balloons. Passive Sensors How Passive Sensors Work. Passive sensors detect military targets either by measuring their natural emission, or by detecting natural light reflected from the targets. A typical sensor is similar to an ordinary camera. An optical element (the lens) forms an image, and alight sensitive surface records that image (the film). In BMD infrared sensors, the optical lens would be replaced by a system of reflecting mirrors and the camera film by an array of discrete optical detectors in the focal plane which convert the optical image into electronic signals for immediate computer processing. Many detectors are required to record a detailed image. In a sense each detector substitutes for one grain of photographic film. Some sensors use a stationary two-dimensional staring array of detectors, in direct analogy to photographic film. Others mechanically scan the image across an array of detectors that may be either two-dimensional or linear. Infrared Sensors. Ordinary photographic cameras record the visible light reflected from a scene. For BMD, the IR energy emitted by the target (particularly the hot exhaust gases ejected from a missile booster engine) is abetter source of information. 18 The sensor images the infrared radiation from the target and background onto a photosensitive array of detectors. These detectors generate a series of electrical signals that are processed by computers to detect and track the target. There are three distinct target classes for the BMD mission: missiles with their rocket engines firing, post-boost vehicles with much lower power engines, and cold objects such as RVs and decoys in space. Each type of target demands different IR sensors. Hot exhaust gas from a booster engine radiates primarily in relatively narrow bands of short wavelength IR. The exact wavelength of this radiation is Photo credit: U.S. Department of Defense, Strategic Defense Initiative Organization Infrared image of the moon from SDIO s Delta 181 experiment. That experiment took measurements of a rocket booster and other objects in space to gather information about the kinds of sensors that would be needed in a space-based ballistic missile defense system. This may be the first long-wave infrared image acquired from a platform in space. aall objects with a temperature above absolute zero( 273 C) emit energy in the form of electromagnetic waves, such as light waves, infrared waves, microwaves, etc. For example, the human body continuously radiates infrared waves. To an infrared camera, we all glow in the dark : our bodies would be recorded on infrared film as a group of hot spots, even if the picture were taken in absolute darkness. Similarly, any target emits energy which can, in principle, be detected with appropriate sensors, provided only that the target is warmer (or colder) than the background scene. l~he RVS do heat up from friction as they enter the atmosphere.

93 86 determined by the particular gas constituents. The primary emission bands for gas plumes are near the water vapor and carbon dioxide lines at 2.7 micrometers 20 (in the short wave IR or SWIR) and at 4.26 µm (in the middle wave IR or MWIR). 21 Other specific radiation lines may help identify some Soviet booster plumes: this will be investigated in the SDI research program. These plumes radiate hundreds of thousands to millions of watts per steradian (W/sr) of energy. Post-boost vehicles also have propulsion systems, but their smaller motors radiate only hundreds of W/sr. Reentry vehicles remain near room temperature (20 o C or 2930 K) in mid-course, until they are heated by the friction of the atmosphere on reentry. The maximum radiation for room temperature objects is near 10 µm in the LWIR. Infrared detection of RVs is difficult because of their low level of radiation (typically a few W/sr) and poor contrast against the earth background. That is, the earth is also near room temperature, with strong emission in the 10-pm band. An IR sensor cannot see a red target against a red background. The sensor would generally have to wait until the target RV was above the horizon to view it against the cold (4 o K) temperature of space. The sensor system would also have to filter out the IR energy from planets or bright stars in the field of view. 22 The technical feasibility of detecting relatively cold RVs against a space background was demonstrated on June 10, 1984, when an LWIR sensor on board the Army s Homing Overlay Experiment (HOE) missile successfully detected a simulated RV over the Pacific. --- ~oone ~crometer (w) is one millionth (10 o) of a meter. ZIA~wpheric water vapor and carbon dioxide attenuate most of the IR radiation from a missile plume in the early stages of flight. However, the higher temperature and pressure of the water and CO* in the plume produce a broader IR spectrum than the atmospheric absorption bands. Infrared energy will therefore leak through on both sides of the 2.7 and 4.3pm lines, even from rockets close to the surface of the Earth. The Air Force has used a star as the target for tests of the U.S. F-15 launched ASAT, which uses a LWIR sensor to home on its target. Ocean. 23 The sensor guided the HOE projectile into a collision course, destroying a target launched earlier from Vandenberg AFB in California. This test demonstrated an ability to detect and track a single approaching RV in space at relatively close range. (The initial HOE missile trajectory was specified by radar signals from Kwajalein until the missile LWIR sensor could acquire the target.) Tracking thousands of RVs and possibly hundreds of thousands of decoys with spacebased sensor satellites from distances of 5,000 to 10,000 km would be more challenging, particularly if the RVs were encapsulated in balloons and decoy balloons were tied (tethered) together or to an RV. Three-Color Infrared Sensors. Depending on the offense s countermeasures, discrimination of RVs from decoys might be improved if the object temperatures could be measured accurately. Long-wave IR sensors that detect one narrow wavelength band cannot determine temperature. That is, a warm object with low IR emissivity 24 could produce the same radiance at one wavelength as a cooler object with high emissivity, as illustrated in figure 4-5. However, the shape of the blackbody (nonreflecting object) radiation curve as a function of wavelength is distinct for objects at different temperatures. This suggests that two or more LWIR sensors operating at different wavelength bands within the 8- to 24-µm region could estimate the temperature ofµpace objects, independent of their general emissivities. Most SDI architects recommended threecolor LWIR detectors to measure energy in three separate wavelength bands or colors. Note that this complicates sensor design and 23 To place this experiment in perspective, it should be noted that this RV was significantly brighter than the radiance expected from current RVs, while the Soviets may take steps to further reduce IR emissions. 24 The emissivity of any object indicates its ability to radiate energy. Emissivity is defined as the ratio of the energy radiated at any wavelength to the amount of energy radiated by a perfect blackbody at the same temperature. (A blackbody absorbs all energy reaching its surface.) Thus an object with low emissivity will radiate less energy than a higher emissivity object, even though they are both at the same temperature.

94 87 Figure 4-5.-Spectral Response of Two Objects at Different Temperatures I Photo credit: U.S. Department of Defense, Strategic Defense Initiative Organization Cryocooler for space applications. Many of the advanced heat-detecting infrared sensors necessary to identify and track missiles and warheads in space must be cooled to work properly. Special refrigerators called cryocoolers would produce the needed very low temperatures. Cryocooler life, reliability, and performance experiments designed to demonstrate the ability to cool long-wave infrared detectors have been conducted. of 7 years, and at least one type of cryogenic refrigerator has demonstrated this ability in accelerated life tests. 25 UN/Visible Sensors. Some SDI contractors have proposed the use of visible or even ultraviolet (UV) sensors, primarily to achieve better resolution with realistic optics dimensions. 26 For example, a 28-cm diameter UV mirror at 0.3 pm could achieve the same resolution as a 400-cm (4-m) diameter mirror operating at 4.3 µm. However, this gain is not free: reducing the wavelength increases the fabrication difficulty. Mirrors must be polished to within one-tenth to one-twentieth of the operating wavelength. Thus an MWIR mirror at 4.3 µm must be polished to within at least 0.43 µm of the prescribed surface figure, while a UV mirror must be polished to an accuracy of 0.03 µm or better. b Hughes Aircraft has demonstrated operation of a magnetic gas cooler system with an accelerated test simulating 7 years iife. ZGThe resolution of a sensor is limited by diffraction spreading of the optical image. This diffraction spreading is proportional to the wavelength of light used to form the image; shorter wavelengths produce less image spreading, yielding better res~ lution or sharper images.

95 88 Visible or UV sensors might detect energy from rocket plumes, although the visible radiation from liquid-fueled missiles is minimal. The atmosphere attenuates UV below an altitude of a few tens of km, but a post-boost vehicle propulsion system may generate adequate UV radiation. To see RVs, however, these sensors would have to rely on the reflection of natural radiation (sunlight, moonlight, or Earthlight). Alternatively, they could be used in an active mode with a laser designator illuminating the target (see next section). Current Status of Passive Sensors. Passive infrared sensors operate today in early warning satellites. A few satellites at geosynchronous orbit, some 36,000 km above the earth, monitor the entire globe, searching for missile launches from the Soviet land mass or from the oceans. Several heat-seeking tactical missiles such as the air-to-air Sidewinder and the ground-to-air Maverick missile also employ infrared sensors. This same sensor technology supplied the terminal guidance for two successful space hit-to-kill experiments: the antisatellite (ASAT) experiment in which a missile fired from an F-15 aircraft destroyed a satellite in space and the Homing Overlay Experiment. Today s operational infrared sensors have relatively small optical systems, typically 20 cm or less in diameter, and focal plane arrays of a few thousand detectors. Most detectors are fabricated from bulk silicon and could not survive in a nuclear environment. Relatively few large detector arrays are built each year, and the United States does not yet have the manufacturing technology to build large arrays economically. Key Issues for Passive Sensors. This report has identified five key issues for passive sensor technology development (see table 4-l). While driven by the space-based system requirements, these same sensor functions would be required for effective ground-launched weapons systems. Whether the sensors rode on airborne or space-based platforms, these issues would have to be resolved to produce a robust BMD system. Mirror Size. A sensor system mirror must be large to collect enough energy, to resolve closely spaced objects, and to accurately direct weapons systems (see box 4-A). The mirror size needed is determined by sensor operating wavelength, distance to target, and target positional accuracy required by the weapon system. The resolution of any optical system is given approximately by the wavelength divided by the diameter of the aperture multiplied by the range. Typical mirror sizes for adequate spot resolution from a passive sensor at 3,000 km altitude are shown in figure To provide adequate aiming information to homing kinetic energy weapons, sensor resolutions from 10 m ZTFig. 4.6 assumes a perfect, diffraction limited OptiCd system. In practice other factors-such as vibration, i.nqxwfect mirror quality, and thermal distortions-would degrade resolution. This figure, therefore, represents the minimum allowable mirror size for a spot. Tracking resolution may only require mirrors a factor of 10 smaller, as noted in the text. Table 4-2. Key Issues for Passive Sensors KEW DEW Current status Mirror size (m) about 0.1 about Number of detector elements (UV/visible) (resolution limited) Geo/staring N/A many tens of thousands Geo/scanning N/A 3,000 km/staring (1 o FOV) ,000 km/scanning Detector manufacturing capacity /yr /yr IO 6 /yr Signal processing Rates % /s several x 10 7 /s Memory X x x 10 7 SOURCE: Office of Technology Assessment, 1988.

96 The resolution of any electromagnetic sensor (or its ability to separate two closely spaced objects) is limited by two factors: diffraction and detector element size. The image formed by the sensor optics cannot faithfully reproduce the actual scene. An infinitesimally small point in the scene will have a finite size in the image due to diffraction or spreading of the light beam. This spreading increases with distance, so diffraction will limit the useful range of any sensor as shown in figure 4-6a. The optical system projects an image of the scene onto the detector array. The size of each.. -- Box 4-A. Sensor Resolution Limits detector element in this array must be equal to or preferably smaller than the optical resolution size to preserve the diffraction-resolution of the figure in the electronic signal. If the detector elements are too large, then they will further limit the system resolution. For a fixed field-of-view, as the distance between the scene and the sensor increases, then each detector element covers a larger area in space: the resolution decreases with range, the same dependence as diffraction spreading of the optical image. 89 Figure 4-6a.-Diffractlon-Llmited Range for Ten-Meter Resolution Figure 4-6b. - Range Limited by Number of Detectors for Ten-Meter Resolution Mirror diameter (meters) Sensor range as a function of mirror diameter to produce a 10-meter resolution element at the target, for three different wavelength sensors. Two point targets separated by 10 meters at these ranges could just be resolved by mirrors of these sizes. Range of LWIR sensors as limited by the number of detector elements in the focal plane array. The staring array is a fixed, two-dimensional array with a 200 field-of-view. The scanning array covers a 10 by 360 coolie hat pattern, with 10 rows of elements scanning each point in the image. Both arrays detect three different LWIR bands. The scanning array could use just one row of detectors to sweep out the image. However, to improve signal-to-noise ratio, most designs utilize more than one row and time delay and integrate (TDI) circuits to average the signals from many rows. Track resolution, however, imposes a less stringent requirement than the spot resolution for a single look. Data from many looks can be combined, using statistical techniques, to achieve up to a tenfold improvement. There- fore, proportionately smaller mirrors are needed for predicting tracks. Directed-energy weapons would require much better resolution than SBIs, since they up to 1 km maybe adequate, depending upon the sensors and the divert capability of-the interceptor. As shown in figure 4-7, mirrors of l-m diameter or less are adequate for any visible or IR wavelength. Furthermore, a l-m mirror operating at 2.7 µm would yield 10-m target accuracy from 3,000 km The primary water vapor emission line from missile exhaust plumes is at 2.7 pm.

97 90 Figure 4-7. Mirror Size Plotted v. the Operating Wavelength of a Sensor System 1, Wavelength (mlc ens) FEL NPB Mirror size plotted v. the operating wavelength of the sensor system, assuming a 3,000 km range to the most distant target, for Indicated spot resolution. Note that the tracking resolution can be up to a factor of 10 better than the resolution calculated for one look, based on diffraction limits. Therefore, the tracking may only require mirrors up to 10 times smaller than indicated in the figure. For homing kinetic energy weapons, moderate-sized mirrors (well under 1 meter in diameter) would be adequate for all wavelengths. Directed-energy weapons such as high power lasers would require sensors with very large mirrors operating in the visible or even ultraviolet region of the spectrum. Thus all DEWS would have to use a Iow-resolution LWIR sensor to point a second UV/visible active sensor or laser on each weapons platform to achieve the necessary accuracy. SOURCE Off Ice of Technology Assessment, must be focused to a small spot without the benefit of a homing sensor at close range. LWIR sensor mirrors to direct DEWS would have to exceed 10 m in diameter. Therefore a DEW sensor would probably have to operate in the SWIR or MWIR, visible, or even ultraviolet (UV) wavelengths. 29 Laser beam weapons would demand the highest accuracy to take full advantage of their small spot size and therefore high intensity on target, typically on the order of 30 cm at 3,000 km or 0.1 microradian. Neutral particle beams, as currently envisaged, would have about one microradian 29 This might be satisfactory for boost-phase kills, but cold RV s in mid-course could only be detected with LWIR sensors. Hence a future laser BMD system designed to attack RV s would have to use a coarse LWIR sensor for detection, then a separate laser designator at shorter wavelength to illuminate targets for tracking by a second UV or visible-light sensor. This complexity, combined with the durability of RV s as a result of their ablative shield needed for reentry, makes the use of laser beams for killing RV s in mid-course very doubtful. divergence, producing a 3 m spot at 3,000 km, so NPB sensors could be about 10 times less accurate than laser beam sensors. Number of Detector Elements per Array. Each passive sensor would need many detector elements for both adequate resolution and high signal-to-noise ratios. For example, a staring array sensor on a BSTS satellite at geosynchronous orbit (36,000 km) could need well over a million detector elements to afford coarse resolution at the surface of the Earth. This requirement could be reduced to hundreds of thousands of detector elements by scanning the IR image over a smaller array of detectors, so that each detector sampled many resolution elements in the IR image. Many detector elements would also be necessary to yield adequate signal-to-noise ratios: the electrical signal produced by IR radiation from a target would have to exceed the signal from all sources of noise. Competing IR noise could come from the background scene such as the Earth or stars, from the mirrors and housing of the sensor system, and from the internal electrical noise of the detector elements. The signal-to-background-noise ratio could be maximized by distributing the background from a fixed field-of-view over many detector elements. 30 For the most stressing task of detecting cold RVs above the horizon against atmospheric background at a tangent height of 50 to 80 km, sensors would need at least several hundred thousand detector elements to generate adequate signal-to-noise ratios. 31 Current IR focal plane arrays on operational military sensors for tactical elements have up to 180 detector elements. Some other operational systems have several thousand, and experimental arrays with many more than 10, Ideally, each detector element should be the same size as that of the target image. If the elements were twice this ideal size (half the total number of detectors in the array), then each element would collect twice the background noise with no increase in signal: the signal-to-noise ratio would be cut in half. For many long-range BMD missions, the detector element would be much larger than the target image. slthese num~rs of detectors are based on the assumption that the sensor mirrors are cooled to the800 to 1000 K range so that IR radiation from those mirrors does not dominate the noise, and that the detectors are fabricated with low noise.

98 91 Photo credit: Genera/ Electric Company Sensor focal plane array of 128 by 128 detector elements. These elements convert light energy into electrical signals. Focal plane arrays are the electrooptical equivalent of film in a camera. Some SDI sensors may require focal planes containing hundreds of thousands of detector elements. elements have been fabricated. The focal plane array (FPA) for the planned Airborne Optical Adjunct (AOA) experiment will have a 38,400- element three-color FPA. 32 However, none of these detectors was designed to the radiation hardness needed for BMD sensors See Aviation Week and Space Technology, Nov. 10, 1986, P. 87. Detector Radiation Hardness. Ballistic missile defense sensors must withstand radiation from distant nuclear explosions. Current detectors are fabricated from relatively thick bulk materials such as silicon or mercury cadmium telluride (HgCdTe) which are susceptible to radiation damage. Other materials, such as gallium arsenide or germanium, or thinner detector structures would be needed to achieve radiation hardness goals. Impurity band conductor (IBC) detectors, which are only 10 to 12 µm thick, can withstand 10 to 100 times more radiation than common bulk silicon detectors. Arrays with up to 500 IBC elements have been fabricated in the laboratory. The electronic readout from FPAs must also be resistant to radiation damage. In the past, charge-coupled devices (CCD) were used to read out large detector arrays. To reduce susceptibility to radiation damage, researchers are butt-bonding switching metal oxide semiconductor field effect transistor (MOSFET) readouts to the detectors. Detector Manufacturing Capacity. Industry produces about 1 million IR detectors per year. Many of these are small linear arrays of 16 to 180 elements each, used for tactical IR missiles or scanning IR imaging systems. The Teal Ruby 33 experiment bulk-silicon array is the largest built so far. Production would have to increase by one or two orders of magnitude to satisfy the ambitious BMD goals: very large, radiation-hard, low-noise arrays would be required. For example, just one BMD sensor would require several, perhaps up to 10, times the current annual production capacity and there could be many tens of sensors in a second-phase space-based BMD system. The SDIO has programs underway intended to gteal Ruby is an experimental satellite designed to detect aircraft from space with an LWIR detector array. Photo credit: U.S. Department of Defense, Strategic Defense Initiative Organization Impurity Band Conduction Long-Wave Infrared Detector Array

99 92 achieve these improvements in manufacturing capability. Conversion from laboratory fabrication to full-scale manufacturing of the new IBC detectors-assuming they continue to be the preferred detector could limit BMD sensor deployment. Industry Performance in converting to the manufacture of bulk silicon IR common module arrays in the early 1980s was not good. Producing arrays of just 60, 120, or 180 elements once held up the completion of M-1 tanks that use forward looking IR (FLIR) sensors. Manufacturing yield (the ratio of the number of acceptable arrays to the number manufactured) for IR detectors would have to be improved. The overall yield (including read-out) for the Teal Ruby array was about 2 percent. Since yield was so low, every element had to be individually tested at cryogenic (10 K) temperatures: testing might be the limiting manufacturing process. The SDIO has initiated programs to address this problem in fiscal year Signal Processing Improvements. Projected signal processing rates for BMD sensors would exceed current space-ebased operational capabilities by factors of a few hundred. Current operational signal processors can handle up tens of millions operations per second (MOPS), while BMD signal processing requirements might exceed 10 billion operations per second, or 10 giga-ops (GOPS). Projected on-board memory requirements for BMD sensors vary from 10 million to 100 million bytes of information. Reaching these memory and processing goals by the 1990s seems likely, given the progress in very high speed integrated circuits (VHSIC). Power consumption of signal processors must be reduced. The AOA experiment will require less than 10 kilowatts (KW) of power to drive a 15 GOP processor, or over 1.5 MOPS/W. Hardened VHSIC technology offers the promise of many times less power consumption (40 MOPS/W) and good radiation resistance. Active Sensors How Active Sensors Work. Active sensors illuminate the target with radiation and monitor reflected energy. In general, active sensors have the advantage of adequate illumination under all conditions: they do not have to rely on radiation from the target or favorable natural lighting conditions. They suffer the disadvantage, under some circumstances, of being susceptible to jamming or spoofing: the opponent can monitor the illumination beam and retransmit a modified beam at the same frequency to overpower or confuse the receiver. At the very least, the illumination beam can alert the enemy that he is under surveillance or attack. This might be a concern for surveillance and tracking of defense suppression weapons such as direct-ascent or orbiting ASATs. Microwave radar, an active sensor used so successfully in tracking aircraft, might support some phases of BMD, particularly for terminal defense. These ground-based radars might use advanced data processing techniques to generate pseudo-images of RVs to distinguish between RVs and decoys, as described below. Conventional microwave radar has two serious limitations for most spacebased BMD functions: limited resolution and large power requirements. Because of the large antennae, large power requirements, and survivability issues, microwave radar is not a prime candidate for BMD space applications. 34 However, the SDIO still believes that microwave radar might be included in future BMD systems. SDI researchers are also investigating laser radar or ladar for applications such as measuring the range to a target and discriminating RVs from decoys. In principle, ladar is equivalent to radar with much shorter (opti me SDIO had considered developing shorter millimeter wave radar to provide better radar resolution and lower power require ments. With reduced funding, support for millimeter radar has been reduced. Distributed antenna arrays are also being considered to provide space surveillance of aircraft and cruise missiles for the Air Defense Initiative.

100 93 cal or infrared) wavelengths. With shorter wavelength, ladars generally would give better resolution with less power and weight. Ladars cannot operate in all weather conditions on earth. They are therefore better suited for space applications. Imaging Radars. If an object is moving relative to a radar, then the radar return signal is shifted in frequency, similar to the Doppler frequency shift of a train whistle as it passes by a stationary observer. For objects that rotate, such as spinning satellites or reentry vehicles, pseudo-images can be generated by processing the doppler frequency shifts of radar signals stored over time. This is a process similar to synthetic aperture radar, sometimes called inverse synthetic aperture radar (ISAR). 35 Consider a conical RV spinning about its axis (figure 4-8). The tip of the cone has no significant motion due to rotation, and little doppler frequency shift. The back edge of the cone has a large motion (proportional to the radius of the cone and the angular velocity of the RV) and a large doppler frequency shift. A plot of range to target versus doppler frequency shift will therefore resemble the shape of the RV for most orientations of radar beam to spinning RV. 36 The resolution of range/doppler pseudoimages does not depend on radar-beam spot size. The beam floods the target area, so precise beam pointing is unnecessary. Range resolution is inversely proportional to the band ~5An ~r~me synthetic aperture radar system generates ~ image of the ground by measuring the doppler frequency shifts of ail return radar signals. Targets directly ahead of the radar aircraft have maximum Doppler frequency shift because the relative velocity between the ground and the aircraft is a maximum. Targets perpendicular to the aircraft flight path have no relative motion toward the aircraft and no Doppler frequency shift. By storing all the radar returns and processing data over time, a pseudo image of the ground is generated. *If ~ fia~g radm were boresighted aiong the trajectory of an RV, there would be no doppler tiquency shift and no image. Conversely, if the radar looked perpendicular to the RV flight path, there would be no information on the length of the RV: any range spread would be due to the radius of the cone, independent of length. For other radar look angles between these extremes, the doppler frequency shift would be proportional to the sine of the look angle, and the range spread would be proportional to the cosine of that angle. Figure W.-l Illustration of an Imaging Radar Viewing a Spinning Conical Target Spinning cone in space Radar beam I Range/doppler pseudo image on radar screen Point A on the base of the cone has the most motion toward the radar, producing the largest doppler frequency shift. The echo from this point would appear at point A on a radar-generated plot of range versus doppler frequency shift. Point B, at about the same range as point A, is moving perpendicular to the radar beam, and will have no doppler frequency shift; its echo would be plotted as shown. Similarly, point C is moving away from the radar, and would have a negative doppler frequency shift. Finally, points along the cone such as point D n have lower frequency shifts, since they are closer to the spinning axis. The resulting range-doppler plot will therefore resemble the conical target. width of the transmitted signal. For example, a one gigahertz 37 bandwidth radar signal could have a range resolution capability of 15 cm. Resolution in the cross-track direction (corresponding to the radius of the spinning cone) is limited by the minimum doppler frequency shift that can be detected, radar wavelength (smaller is better), and the rotation rate of the RV (larger is better). 38 For microwave radars, typical doppler frequency shifts are in the tens to hundreds of hertz. Many radar pulses must be stored and analyzed to measure these low frequencies, which requires substantial data processing... ~TGig~e~z is a unit of frequency equal to one billion cycles per second. anote that doppler (cross-track) resolution of these pseudo images is not equivalent to positiomd accuracy. Object details on the order of a few cm may be resolved in these images, but the cross-track position of the object will not be known to better than the radar beam width, which might be tens of kms wide.

101 Ladar. The short wavelength and very short pulse-length of a laser might prove very useful for several BMD functions. A laser radar or ladar system would illuminate the target cloud with a pulsed beam of light. An optical receiver would detect the reflected echoes, in direct analogy to a microwave radar. Various types of ladars could supply one-dimensional range to the target (a laser range-finder), or they could generate 2- or 3-dimensional images. Several modes of imaging operation are possible: Scanning beam or angle/angle mode: a pulsed laser beam is focused and scanned over the scene. A single optical detector records the time sequence of reflections from each returned laser pulse, and a three dimensional map of target position is generated in computer memory. Ladar resolution would depend on the beam spot size, which could be as small as 3 m at 3,000 km with reasonably sized optics. 39 Very short-wavelength lasers are preferred to minimize spot size. The range resolution would be on the order of 1.5 m with 10- nanosecond long laser pulses, which are commercially available. 99A 0.5 ~m laser with a 60-cm mirror would produce a diffraction-limited spot 3 m in diameter at a distance of 3,000 km. Focal plane array: a passive imager, similar to the IR sensors, records the scene illuminated by a laser. The laser is the flash lamp. Doppler ladar: the optical analog of a microwave Doppler imaging radar might be feasible if lasers with adequate coherence could be built. Doppler resolution of a coherent ladar could be excellent. A 30- cm RV rotating once per second would generate a 3.8 megahertz (million cycles/ second MHz) frequency shift in the ladar return signal, compared to only 60 hertz for an X-band imaging radar. Since the resolution of this pseudo-image would be independent of spot size, there would be no need to operate at short UV or visible wavelengths. This fine image resolution would not, however, yield good positional information. A narrow beam (short wavelength) angle/angle ladar would be required for good angular resolution. Active Discrimination. A ladar might be very useful for discriminating between RVs and decoys as they were ejected from a PBV. The PBV would perceptibly change its velocity as each heavy RV was discharged, but not as light decoys were dispensed. A ladar could be designed with the spatial resolution to resolve independently the PBV and the RV or decoy and, in theory, to measure the differen-

102 95 tial velocities before and after each deployment. 40 Light decoys might inflate as they left the PBV. A high resolution imaging ladar could in principle observe this inflation and so identify balloon decoys. 41 A precision doppler ladar might also observe small vibration or nutation (wobbling) differences between an RV and a decoy. Light decoys might vibrate at tens to hundreds of kilohertz (khz), heavier RVs at less than a few khz. Over tens of seconds, the nose of a spinning RV also nutates a few millimeters: a very high resolution ladar might detect this motion, but long integration times and high data storage rates would be necessary. Current Status of Active Sensors. Active sensor technologies have been tested and deployed in some form since the radars of World War II. Considerable development remains, however, before active sensors will be ready for advanced BMD systems. Phased-array Search Radars. -Ground-based phased-array radars are currently deployed in both the United States and the Soviet Union to detect objects in space and give early warning of missile attack. The PAVE PAWS radars now at Otis AFB on Cape Cod and at Beale AFB near Sacramento have two large faces each, with active areas 22 m square, providing 2400 coverage. Each face has 1,792 active antenna elements, with provisions to upgrade each face to 31 by 31 m active areas with 5,354 elements. Two additional PAVE PAWS radars are being built in Georgia and Texas. 40 Consider a PBV with 10 RVs. The PBV velocity would change very little if a light decoy were ejected. Ejecting the first RV, if it weighed l/15th of the remairing PBV weight, would cause the PBV to slow by l/15th of the RV-PBV separation velocity. That is, if the two objects were designed to move apart at a 15cm/sec rate, then the PBV would slow down by 1 cm-dsec and the RV would speed up by 14 cndsec after separation. Later RV S would cause the PBV to slow down more, as the ratio of RV to remaining PBV weight increased. The ladar would therefore need a velocity resolution of 1 cndsec in this example. ilone Countermeasure to block the observation of decoy inflation (as well as differential velocity detection) would be to inflate the decoys under a long shroud, although there is some concern that the PBV rocket plume might interfere with a shroud. Alternatively, decoys and RVS could be tethered together so that their rotation would confuse the sensor, which could not keep track of each object (see ch. 10.) The United States plans to replace the three existing Ballistic Missile Early Warning System (BMEWS) mechanically scanned radars at Clear, Alaska; Thule, Greenland; and Fylingdales Moor in England with phased array radars. The old Distant Early Warning radars will also be replaced by 52 new phased array North Warning System (NWS) radars. These radars, along with the mothballed phased array radar near Grand Forks, North Dakota, might supply RV target coordinates to an ERIS exoatmospheric interceptor system. 42 ImagingRadars. Several radars have been operated in the rangedoppler imaging mode since the early 1970s. These ground-based radars are used to image satellites, RVs, and other space objects. MIT s Lincoln Labs operates an L-band and an X-band imaging radar at Millstone Hill in Massachusetts. Ladars. Ladar systems have not been placed in operation, but they have been tested. In 1981 MIT Lincoln Laboratories built the Firepond CO, ladar, which had a 15 kw peak power and 1.4 kw average power. With a one microradian resolution, this ladar could detect targets spaced 3 m apart at a distance of 3,000 km. This ladar has been reactivated for the SDI program. It will be operated in the range-doppler mode to investigate RV imaging in a ground-based field test. Two other lasers are planned. One will have a very short (nanosecond), high peak power pulse to yield good range resolution. The other will use a lower peak power, frequency-chirped pulse. To recover good range resolution, this chirped pulse is compressed electronically in a data processor. This same pulse compression technique has been used successfully to reduce the peak power required in more conventional microwave radars. z SDIO s phaseone Strategic Defense System pkms one or more optical sensors for cueing ERIS interceptors. However, Lockheed-the ERIS developer and others have proposed an early deployment version of ERIS that would utilize existing radars. The computing capabilities of these radars wouid have to be improved to handle hundreds of targets. The systems would be susceptible to electromagnetic pulse, microwave jamming, and blast damage in the event of nuclear war. At this time, phased-arra y radars are the only sensors available for early deployment of ERIS-like BMD systems.

103 Work is also proceeding on diode-pumped glass lasers, excimer lasers, and bistatic CO 2 ladars. Glass lasers are typically pumped with flash lamps, resulting in very low efficiency (typically less than 0.2 percent), since the spectrum of the flash lamp does not match the absorption bands of the Nd:glass material. By pumping the Nd:glass laser with an array of incoherent laser diodes, efficiency can be increased significantly and the thermal distortion which normally limits these lasers to very low repetition rates can be controlled. Excimer lasers have the advantage of generating UV radiation, which demands the smallest mirrors for a given resolution. Key Issues for Active Sensors. Current SDI phase-two concepts call for ground-based radars for directing late mid-course and terminal defense. Space-based ladars are suggested for boost-phase ranging, to observe PBV deployment, and for determining accurate target position during mid-course discrimination. Ladars might also be used for air-borne ranging to assist terminal defense. Issues for these active sensors include the following. Ground-based Radar.-Ground-based radars would have to be large, phased-array devices to focus adequate energy on many targets. Two key issues would be survivability and data processing. Surge fuses at each radiating diode in the array could probably protect large antennas from nuclear burst-generated electromagnetic pulse (EMP). Shielding the structure and building could protect interior electronics. Most EMP energy would be below 150 MHz, so radar radio frequency (RF) circuits at 10 GHz could be safe. However, these antennas would be susceptible to in-band radiation from dedicated jammers. It might be a challenge to design effective electronic counter-countermeasures to protect these large and critical assets from electronic jamming by Soviet satellites. Some system architects have suggested that these radars be mobile, possibly on railroad cars. Mobility might reduce susceptibility to jamming. Data processing might also be challenging. Consider an X-band (3-cm wavelength) radar. Its data processor might have to handle 5 million bits per second of incoming data for each of 5,000 antenna dipoles, or a total of 25 billion bits per second for the entire radar. 43 These data must be stored and processed to determine the direction to each target (by phasing the receiving array) and a Fast Fourier Transform (FFT) operation would have to be performed on each range bin to measure doppler frequency shift over many pulses. Doppler imaging radars might be fooled if RVs (and decoys) were covered with fronds, strips painted with irregular patterns of volatile material. Attached at various places on an object, these strips would move about at random in space as the volatile material evaporated. This motion would give different parts of the target different doppler velocities independent of their positions on the RV or decoy cone. Such extraneous frequency shifts might confuse the radar processor, obscuring the image of the RV body. Ladar Active Discrimination. Significant advances would be required in ladar technology before it could be utilized to observe PBV deployment of RVs and decoys. Key issues would be resolution, beam steering, and data processing to handle the expected traffic. Direct angle/angle ladar imaging of PBVs would take very large mirrors. 44 The alternative would be doppler processing to improve cross-track resolution. While microwave syn- 4~his data rate assumes that radar bandwidth is 1 GHz to yield a 15-cm range resolution. The radar tracks each target to within 100-m accuracy before hand-over to an image mode Processor, which maintains a shiingrange gati 100 m wide about each high-speed target. The radar pulse repetition rate is set by the highest expected doppler frequency shift produced by RV rotation. For clear images of a 20-cm radius RV rotating at 3 hertz, the pulse rewtition frequency (PRF) must be 500 hertz or higher. (This imaging doppler radar would be highly ambiguous with respect to RV velocity, which would require MHz type PRFs to measure actual velocity.) 44 T0 image a 30-cm diameter RV, a ladar designer would like 10 resolution elements across the object to resolve shape or details, or 3 cm resolution. Thus, an impractically large 60-m mirror would be required for 3 cm resolution at 3,000 km range with a visible laser.

104 97 thetic aperture radars have been successfully operated for over 20 years, this process has not been extended to optical wavelengths. Building stable but powerful space-based lasers with the coherence necessary for doppler processing would be a major challenge. Interactive Sensors The consensus in the SDI technical community is that passive and active sensors may not be adequate to discriminate between RVs and decoys in the future. The Soviet Union probably has the necessary technology to develop decoys and real RVs with nearly the same infrared and radar signatures. Decoys would not be extraordinarily difficult to fabricate and disperse in space, and they would weigh only a small fraction of an RV. There is a serious question whether, once dispersed, they could be distinguished from real RVs by any passive or active sensor. If not, the offense could overwhelm a space-based or ground-based midcourse defense system with literally hundreds of thousands of false targets. Mid-course decoy discrimination would become crucial if the Soviets could: deny a phase-one boost-phase defense through countermeasures such as moderately fast-bum (e.g., 120- second) boosters, and deny significant post-boost kills by moving to faster PBV deployment times or to single warhead missiles. If an initial U.S. deployment of kinetic energy weapons could no longer destroy many ICBMs in the boost or post-boost phase, and if directed-energy weapons were not yet available, then mid-course discrimination would become indispensable to a viable BMD system. There would be two possibilities for effective mid-course discrimination under these circumstances: ladar discrimination during postboost decoy dispersal, or interactive discrimination after the RVs and decoys were released. As discussed in the preceding section, ladar detection during decoy deployment would be very challenging. Moreover, simple measurement of RV and PBV recoil velocities might be thwarted completely if the Soviets could disperse decoys and RVs simultaneously in pairs. Even fine doppler imaging would be foiled if the Soviet PBV could obscure the deployment operation with a shroud. This would leave interactive discrimination as the main approach to keeping BMD viable in the long term. How Interactive Discriminators Would Work. In interactive discrimination, a sensor system would perturb each target and then measure its reaction to determine if it were a decoy or an RV. For example, a dust cloud of sufficient density and uniformity could be placed in front of a group of objects. The resulting collisions would slow down light decoys more than heavy RVs. A ladar would monitor the change of velocity of all objects, thereby identifying real RVs. Two general classes of discriminators have been proposed: kinetic energy and directed energy perturbers. Kinetic Energy Discriminators. Two methods have been proposed to project particles in front of an oncoming cloud of decoys and RVs: rocket-born particles and nuclear-explosionprojected particles. A rocket-borne cloud would be limited to late mid-course, unless the rockets were fired from submarines or based in Canada or the Arctic. Presumably one rocket would be necessary for the cylindrical cluster (or threat tube ) of RVs and decoys emanating from each PBV. To slow down decoys measurably, a rocket would have to carry enough mass to cover the full lateral extent of the threat tube with a sufficiently dense cloud. A ladar would have to measure velocity changes in the 10-cm/sec to l-m/see range. Directed-energy Discriminators. -Several forms of directed energy have been proposed for interactive discrimination. They would all have the advantage of long range, extending the discrimination capability back to the beginning of the mid-course if not to the postboost phase. The laser is the best developed directedenergy perturber currently available, although further development would be needed to produce lasers with the brightness required for

105 98 interactive discrimination. Lasers could heat unknown targets (called thermal tagging ). Alternatively, a short pulse of laser light could change the velocities of targets (called impulse tagging ). In thermal tagging, a laser of the appropriate wavelength would heat a light-weight decoy more than an RV assuming they both absorbed laser energy and radiated IR (thermal) energy to the same degree. A separate IR sensor, possibly mounted on SSTS satellites, would then detect the warmer decoy. Pulsed lasers could shock the unidentified objects. Energy would be deposited in microseconds instead of the milliseconds taken by thermal tagging. A high-power pulse would boil away material perpendicular to the surface of the target. The reaction of ablation products would cause the target to change velocity. A heavier RV would recoil less than a decoy, providing amass-dependent indicator. A separate ladar would monitor the change of each object s velocity. The SDIO has chosen the neutral particle beam (NPB) as the most promising interactive discrimination perturbation source. The particle beam source is derived from well-established particle accelerators used for several decades in physics research experiments around the world. A neutral particle beam could be composed of hydrogen atoms, 45 accelerated to velocities about half that of the speed of light. Since the particle beam would be relatively broad, on the order of 2 microradian beam width, it would not require the pointing accuracy of 50-nanoradian-wide laser beams. These energetic particles would be deposited several cm deep inside an RV. 46 As they were 4SAn NpB could dso utilize deuterium or tritium, the heavier isotopes of hydrogen. These heavier isotopes would experience less divergence in the beam neutralization process after acceleration. Tritium, the hydrogen isotope with two neutrons, must be produced in a nuclear reactor and is radioactive with a halflife of 12.3 years. Deuterium, the non-radioactive hydrogen isotope with one neutron, would most likely be used. Another approach calls for cesium instead of hydrogen atoms in a momentum rich beam. A heavy cesium beam would impart a velocity change to the target, so it is more analogous to a laser impulse tagger than to a hydrogen electron on each hydrogen atom would be stripped off, leaving the proton which penetrates into the target. absorbed, these particles would produce gamma rays and neutrons. Neutron or gammaray detectors on many satellites-located closer to the targets than the accelerator might monitor the emissions coming from a massive RV. Light weight decoys, in contrast, would not emit much radiation. High energy particles must be electrically neutral to propagate through the Earth s variable magnetic field (charged particles would bend in unpredictable paths.) But a particle must be charged to be accelerated. Therefore the NPB would first accelerate negatively charged hydrogen ions. After acceleration to a few hundred MeV (million electron volts) energy, this beam would be aimed toward the target by magnetic steering coils. Once steered, the charged beam would be neutralized by stripping off the extra electron from each particle. Thin foils or gas cells are currently used to neutralize beams in laboratory experiments. A relativistic (i.e., near-speed-of-light) electron beam could also be used as a discriminator. The detector in this scheme would monitor x-rays from the more massive RV. Such a system might be ground-based, popping up on a rocket to monitor the mid-course phase. The main advantage would be the avoidance of space-based assets for interactive discrimination. However, an e-beam discriminator would need some air to form a laser-initiated channel, so it could only operate at altitudes between 80 to 600 km. Current Status of Interactive Sensors. -Interactive sensors have not yet been built for any military mission. All the concepts described above have been invented to solve the severe discrimination problem unique to mid-course ballistic missile defense. Key Issues for Interactive Discrimination. The overriding issue for interactive discrimination is effectiveness in the face of evolving Soviet countermeasures. There are some common issues for any discriminator and some issues unique to each approach. Laser Radar. Any discriminator would require a high resolution laser radar to accurately locate and identify each object in space. One

106 99 Table 4-3. Key Issues for Interactive Sensors For all discriminators: Laser radar required for accurate target location: (corner cube reflector is inexpensive countermeasure.) Rapid retargeting: 3-50 targets/second For For For NPB accelerator: Voltage and duty cycle must be increased without increasing beam emittance Beam expansion Beam sensing must be developed Beam pointing system must be developed Beam propagation in space Space charge accumulation Accelerator arcing in space Weight NPB neutron detectors: RV detection with nuclear precursor background Missed target indicator laser thermal tagger: Moderate to high power pulsed lasers Thermal shroud on RV For laser impulse tagger: Needs Iadar imager to tell orientation High to very high average power, microsecond-long pulsed lasers. Thruster-compensated RVs For dust cloud tagger:. Dispersal of dust cloud SOURCE Office of Technology Assessment, 1988, possible countermeasure to ladar would bean inexpensive corner-cube reflector on each RV and decoy. This corner cube would essentially swamp the ladar receiver: the beam would be returned on itself and the ladar would be unable to measure target characteristics. A counter-countermeasure would be a bistatic ladar with a laser transmitter on one platform and a light detector on a separate satellite not far away. Reflected energy from a corner cube would travel harmlessly back to the transmitter; thus failing to blind the receiver. Bistatic operation would be feasible, but it would complicate system design, construction, and operation. Beam Steering. A directed-energy interactive discriminator would have to steer its beam rapidly from one object to the next. Beam steering requirements are set by the number of expected targets and the number of directedenergy satellites within range of those targets. Typical estimates are that hundreds of thousands of RVs and decoys might survive the boost phase defense. 47 Assuming that midcourse discrimination of sophisticated decoys must be completed in 15 minutes, then each platform would have to interrogate 3 to 50 targets per second. The directed-energy source would have to be steered accurately from one target to the next in less than 20 to 300 milliseconds. This would be a formidable challenge. NPB Accelerator. Neutral particle beam accelerator development faces many key hurdles. Beam energy must be increased by a factor of 20, which should not be difficult. Duty cycle and beam diameter must be increased by a factor of 100 without degrading beam quality or emittance a more challenging task. An accelerator would have to operate in space without electrical breakdown or arcing that would short out its electrical system. Communications and electronic controls would have to operate even with electrical charge buildup in space. An NPB would have to propagate over long distances in space with little divergence. To point accurately at targets, it would have to be effectively boresighted to an optical system. These same issues would have to be resolved for an NPB weapon accelerator. A weapongrade NPB would probably dwell longer on each target to assure destruction of at least the internal electronics, but might otherwise be very similar to one designed for interactive discrimination. A more detailed discussion of NPB accelerator issues appears in the DEW section of chapter 5. Neutron Detection. Calculations indicate that large neutron detectors placed on hundreds of separate satellites near the targets could detect the neutron flux from RVs. The offense might intentionally detonate nuclear weapons in space before an attack to saturate these neutron detectors. With sufficiently high particle-beam energy (on the order of 200 MeV), JTAn ~~ractive discriminator wouid not have to interrogate all objects in space. Unsophisticated decoys, discarded booster stages and other debris could probably be identified by passive or active sensors. With adequate battle management to keep track of extraneous objects, the process of bulk filtering would eliminate these objects from the interactive discriminator s target list.

107 100 the energy of some neutrons ejected from an RV would be higher than that expected of neutrons emanating from nuclear detonations. Therefore an energy threshold circuit would eliminate most of the signal from the latter source, allowing identification of the neutrons from RVs. Another issue is how to confirm that targets had been hit by an NPB, since the neutron detectors would receive no signal from decoys. How would a system distinguish between decoys and RVs which were missed by the beam? One possibility, being tested in the laboratory, would be to monitor each object with a UV sensor on the assumption that the outer surface of the RVs (and the decoys) would emit UV light when struck by the particle beam. This UV sensor simply would confirm that the particle beam had hit a target. If based on current technology, neutrondetector platform weights would be excessive. Each platform would weigh up to 30 tonnes. System designers hope that lighter detector elements and power supplies can reduce this weight to 5 tonnes per platform by the mid- 1990s. If this goal were achieved, then the several hundred detector satellites could be orbited with about 100 launches of the proposed Advanced Launch System. Laser Thermal Tagger. Very high power lasers would be required to tag space targets for an interactive discriminator. A laser thermal tagger, like all interactive sensors, would require a separate laser radar to locate targets precisely. For example, cold RVs (and decoys) would have to be tracked by long-wavelength LWIR passive sensors. These sensors could only determine a target s position to within 18 m, assuming a 2-m sensor mirror at 3,000 km. 48 But the interrogating laser beam might have a spot size of only 1 or 2 m. A more accurate laser radar would be required to guide an HF laser beam to the target. 46A single target could be located to within less than the 18-m LWIR resolution element by a process called beam-splitting : the target is assumed to be in the center of the IR signal wave form. If there were two targets or a target and a decoy within the 18-m resolution element, however, then the sensor would falsely indicate one target located between the two objects. Detecting small temperature rises on several hundred thousand objects would also stress LWIR sensor technology. Monitoring closely spaced targets would demand large LWIR mirrors. For example, to distinguish objects spaced 10 m apart, a sensor 3,000 km away would need a 4-m mirror. Steering this large mirror to, say, 15 targets per second would be another major challenge. Decoys might be modified to respond to thermal tagging as an RV would. Due to their lower mass, decoy surfaces should became hotter than RV surfaces after laser illumination. However, the outer layer of the decoys could in principle be built to absorb less laser light or to emit more IR heat. These decoys would then reach the same temperature as an RV after exposure to laser light. Or, an RV could simply be covered by an insulating blanket that would decouple the exterior thermal response from the internal RV mass. It appears that laser thermal tagging would have limited usefulness against a committed adversary. Laser-impulse Discriminator. The energy density required for laser impulse discrimination would be in the range of 7 to 30 times more than for thermal tagging. In addition, the laser pulses would have to be very short, on the order of microseconds instead of milliseconds, which makes the peak laser power extraordinarily high. This high peak power would be difficult to generate and handle, since mirrors and other optical components would be susceptible to damage by the intense pulses. While less powerful than proposed laser weapons, lasers for impulse discrimination would still be a major development. 49 Laser impulse discrimination might be countered by equipping RVs or decoys to react deceptively. Small thrusters on RVs might cause them to move as a decoy would under a laser impulse. Alternatively, thrusters on relatively sophisticated decoys might counteract the laser impulse. 4YI he primary measure of a laser s effectiveness as a weapon is beam brightness, the average power radiated into a given solid angle. An HF laser impulse tagger would be brighter than any laser built to date, but still a factor of 2 to 200 less bright than that needed for BMD against a responsive Soviet threat.

108 101 All interactive discriminators would probably require an imaging ladar to provide adequate resolution both to hit targets with a probe beam and to measure target response accurately. A laser impulse discriminator would bear the additional burden of determining target (and particularly decoy) orientation. The orientation of a conical decoy, for example, could affect its reactive motion in response to the laser pulse. Dust-cloud Discriminator. The key issue for a dust cloud discriminator is how to position the cloud accurately in front of the oncoming RV-decoy constellation at the proper time. If the particles were dispersed too widely, the required amount would become excessive. If clustered too closely, they could miss some decoys. As with any discriminator, a precision ladar would be required to measure velocity changes accurately. Laser impulse discrimination might be countered by equipping RVs or decoys to react deceptively. Small thrusters on RVs might cause them to move as a decoy would under a laser impulse; alternatively, thrusters on relatively sophisticated decoys might counteract the impulse. SENSOR TECHNOLOGY CONCLUSIONS Phase 1 A boost surveillance and tracking satellite (.BSTS) could most probably be developed by the mid-1990s. Short-wave and middle-wave infrared (S/MWIR) sensors, could provide early warning and coarse booster track data sufficient to direct SBI launches. 50 Space surveillance and tracking system (SSTS) satellites would not be available for tracking individual RVs and decoys before the late 1990s. The ability to discriminate possible decoys in this time frame is in question. Smaller but similar sensors for a phase-one system might be placed on individual SBI platforms or on ground-based, pop-up probes. An airborne optical system could probably be available by the mid-1990s to detect and track RVs and decoys with IR sensors (although not to discriminate against a replica decoy above the atmosphere). However, its utility may be limited in performance and mission: Performance may be limited by the vulnerability and operating cost of its aircraft platform, and IR sensors 600ne ~ncert~nty ig the protection of the BSTS sensors from future airborne or spaceborne laser jarnmers which could permanently darnage IR detector elements during peacetime might be confused during battle by IR-scattering ice crystals formed at 60 to 80 km altitude by debris reentering the atmosphere. The relatively short range of airborne IR sensors would limit the AOS mission to supplying data on approaching objects for endo-atmospheric interceptor radars, and possibly for exe-atmospheric interceptors a short while before RV reentry. Airborne IR sensors, unless very forward-based, could utilize only a small portion of the time available in mid-course for discrimination and therefore could not take full advantage of the fly-outrange of ground-based exoatmospheric interceptors. In any case, an Airborne Optical System is not now included in SDIO phase-one deployment plans. Effective discrimination against more sophisticated decoys and disguised RVs in space is unlikely before the year 2000, if at all. Phase 2 By the late 1990s at the earliest, a space surveillance and tracking system (SSTS) might furnish post-boost vehicle (PBV) and reen-.

109 try vehicle (RV) track data with long-wave infrared (LWIR) above-the-horizon (ATH) sensors suitable for directing SBI launches in the mid-course. New methods would be needed for the manufacture of large quantities of radiation-hardened focal plane arrays. Another issue is the operation of LWIR sensors in the presence of precursor nuclear explosions (including those heaving atmosphere into the ATH field of view) or other intentionally dispersed chemical aerosols. Effective mid-course SBI capability is unlikely before the late 1990s to early 2000s. There are too many uncertainties in projecting sensor capabilities and the level of Soviet countermeasures to specify a discrimination capability for SSTS. It appears that Soviet countermeasures (penetration aids and decoys) could keep ahead of passive IR discrimination techniques: Passive IR discrimination could be available by the mid-1990s, but probably would have marginal utility against determined Soviet countermeasures. Active laser radar (ladar) imaging of PBV deployment offers some promise of decoy discrimination, provided that the Soviets did not mask dispersal of decoys. Space-borne imaging ladars probably would not be available until the late 1990s at the earliest. Laser thermal tagging of RVs is unlikely to be practical given the need for complex, agile steering systems and given likely countermeasures such as thermal insulation of RVs and decoys. Laser impulse tagging is even less likely to succeed in this phase because high-power pulsed lasers would be required. Ground-based radar (GBR) might be available by the late 1990s to direct interceptors to recentering warheads. There maybe some questions about its resistance to RF jammers. Signal processors may have difficulty handling large numbers of targets in real-time. Phase 3 Accurate IR sensors, UV ladar, or visible ladar would have to reside on each DEW platform. Interactive discrimination with neutral particle beams (NPB) appears the most likely candidate to reliably distinguish decoys from RVs, since the particles would penetrate targets, making shielding very difficult. Before one could judge the efficacy of a total NPB discrimination system, major engineering developments would be required in: weight reduction, space transportation, neutral particle beam control and steering, 5l automated accelerator operation in space, and multi-megawatt space power. It is unlikely that a decision on the technical feasibility of NPB discrimination could be made before another decade of laboratory development and major space experiments. Given the magnitude of an NPB/detector satellite constellation, an effective dismimination system against sophisticated decoys and disguised RVs would not likely be fully deployed and available for BMD use until the 2010 to 2015 period at the earliest. Nuclear bomb-projected particles might also form the basis of an effective interactive discriminator, if reliable spacebased ladar systems were also developed and deployed to measure target velocity changes. There are too many uncertainties to project if or when this approach might succeed. Since the particle beams are invisible, novel approaches would be required to sense the direction of the beam so that it could be steered toward the target.

110 Chapter 5 Ballistic Missile Defense Technology: Weapons, Power, Communications, and Space Transportation

111 CONTENTS Page Introduction Weapons...., Kinetic-Energy Weapons (KEW) Directed-Energy Weapons Power and Power Conditioning Space Power Requirements Space Power Generation Technology.143 Power Conditioning Communication Technology GHz Communication Links Laser Communication Links Space Transportation Space Transportation Requirements Space Transportation Alternatives Space Transportation Cost Reduction Conclusions Weapon Technology Conclusions Space Power Conclusions Space Communications Conclusion...l55 Space Transportation Conclusions Figures Figure No. Page 5-1. Orientation of SDI to RV a. Space-Based Interceptor Mass v. Velocity b. Number of Satellites v. SBI Velocity c. Number of Space-Based Interceptors v. Velocity d. Constellation Mass v. SBI Velocity a. Number of Projectiles v. SBI Velocity b. SBI Constellation Mass v. SBI Velocity Total SBI Constellation Mass in Orbit v. Booster Bum Time Boost and Post-Boost Kill Effectiveness (l,400 ICBMs v. Realistic SBIs) a, Boost and Post-Boost Kill Effectiveness (500 single-rv ICBMs at three sites) b a. 5-13b. Table Boost and Post-Boost Kill Effectiveness (500 single-rv ICBMs atone site) Boost and Post-Boost Kill Effectiveness (200 mediumbum-booster ICBMs at one site) Schematic of an Electromagnetic Launcher (EML) or Railgun..119 Lightweight Homing Projectile..l2l Illustration of the Relationships Between Laser Parameters and Power Density Projected on a Target FEL Waveforms Schematic of a Neutral Particle Beam Weapon Annual Space Launch Capacity.152 Space Transportation Tables No. Page Key Issues for Electromagnetic Launchers (EML) Characteristics of a Ground-Based FEL Weapons System Characteristics of an HF Laser Weapons System Characteristics of Directed Energy Weapons Against a Fully Responsive Soviet Threat Possible Beam Steering and Retargeting Requirements for Boost-Phase Engagement Key Issues for Free Electron Lasers (FEL) Key Issues for the HF Chemical Laser Neutral Particle Beam Issues Estimated Power Requirements for Space Assets Current U.S. Space Launch Inventory

112 Ballistic Weapons, Chapter 5 Missile Defense Technology: Power, Communications, and Space Transportation INTRODUCTION weapons, power systems, and communication systems of most interest for the Strategic De- fense Initiative (SD I). Finally, it considers the new space transportation system essential for a space-based defense. This chapter reviews weapon technologies relevant to ballistic missile defense (BMD). It emphasizes the chemically propelled hit-to-kill weapons most likely to form the basis of any future U.S BMD deployment in this century. The chapter also covers the directed-energy WEAPONS A weapon system must transfer a lethal dose of energy from weapon to a target. All existing weapons use some combination of kinetic energy (the energy of motion of a bullet, for example), chemical energy, or nuclear energy to disable the target. The SDI research program is exploring two major new types of weapon systems: directed-energy weapons and ultra-high accuracy and high velocity hit-tokill weapons. Not only have these weapons never been built before, but no weapon of any type has been based in space. Operating many hundreds or thousands of autonomous weapons platforms in space would itself be a major technical challenge. Directed-energy weapons (DEW) would kill their prey without a projectile. Energy would travel through space via a laser beam or a stream of atomic or sub-atomic particles. Speed is the main virtue. A laser could attack an object 1,000 km away in 3 thousandths of a second, while a high-speed rifle-bullet, for example, would have to be fired 16 minutes before impact with such a distant target. Clearly, Note: Complete defi m tions of acronyms and im tial.z sms are h sted in Appen& x B of this report. DEW, if they reach the necessary power levels, would revolutionize ballistic missile defense. DEWS offer the ultimate in delivery speed. But they are not likely to have sufficient deployed power in this century to destroy ballistic missiles, and they certainly could not kill the more durable reentry vehicles (RVs). In hopes of designing a system deployable before the year 2000, the SDI research program has emphasized increased speed and accuracy for the more conventional kinetic-energy weapons (KEW), such as chemically propelled rockets. With speeds in the 4 to 7 km/s range, and with terminal or homing guidance to collide directly with the target, these KEW could kill a significant number of today s ballistic missiles. With sufficient accuracy, they would not require chemical or nuclear explosives. Although DEWS will not be available for highly effective ballistic missile defense during this century, they could play a significant role in an early 1990s decision on whether to deploy any ballistic missile defense system. That is, the deployment decision could hinge on our ability to persuade the Soviets (and ourselves) that defenses would remain viable for the foreseeable future. Kinetic-energy weapons 105

113 106 work initially against the 1990s Soviet missile threat. But Soviet responsive countermeasures might soon render those weapons ineffective. Thus, a long-term commitment to a ballistic missile defense system would imply strong confidence that new developments, such as evolving DEW or evolving discrimination capability, could overcome and keep ahead of any reasonable Soviet response. Strategic Defense Initiative Organization (SDIO) officials argue that perceived future capabilities of DEW might deter the Soviet Union from embarking on a costly defense countermeasures building program; instead, the prospect of offensive capabilities might persuade them to join with the United States in reducing offensive ballistic missiles and moving from an offense-dominated to a defensedominated regime. To foster this dramatic shift in strategic thinking, the evolving defensive system would have to appear less costly and more effective than offensive countermeasures. Today, the immaturity of DEW technology makes any current judgments of its costeffectiveness extremely uncertain. It appears that many years of research and development would be necessary before anyone could state with reasonable confidence whether effective DEW systems could be deployed at lower cost than responsive countermeasures. Given the current state of the art in DEW systems, a wellinformed decision in the mid-1990s to build and deploy highly effective DEW weapons appears unlikely. 1 Kinetic-Energy Weapons (KEW) Today s chemically propelled rockets and sensors could not intercept intercontinental ballistic missiles (ICBMs) or reentry vehicles.- The Study Group of the American Physical Society concluded in their analysis of DEW that even in the best of circumstances, a decade or more of intensive research would be required to provide the technical knowledge needed for an informed decision about the potential effectiveness and survivability of DEW systems, In addition, the important issues of overall system integration and effectiveness depend critically upon information that, to our knowledge, does not yet exist. See American Physical Society, Science and Technology of Directed Energy Weapons: Report of the American Physical Society Study Group, April, 1987, p. 2. (RVs) in space. No currently deployable projectile system has the accuracy or speed to consistently intercept an RV traveling at 7 km/s at ranges of hundreds or thousands of kilometers. The SAFEGUARD anti-ballistic missile (ABM) system built near Grand Forks, North Dakota in the early 1970s, and the existing Soviet Galosh ABM system around Moscow both would compensate for the poor accuracy of their radar guidance systems by exploding nuclear warheads. The radiation from that explosion would increase the lethal radius so that the interceptors, despite their poor accuracy, could disable incoming warheads. The goal of the SDI, however, is primarily to investigate technology for a non-nuclear defense. This would dictate the development of smart projectiles that could see their targets or receive external guidance signals, changing course during flight to collide with the targets. The following sections discuss proposed KEW systems, KEW technologies, the current status of technology, and key issues. KEW Systems Four different KEW systems were analyzed by SDI system architects, including spacebased interceptors (SBIs, formerly called space-based kinetic kill vehicles or SBKKVs), and three ground-based systems. All four systems would rely on chemically propelled rockets. Space-Based Interceptors (SBIs). Each system architect proposed-and the SDIO phase one proposal includes deploying some type of space-based projectile. These projectiles would ride on pre-positioned platforms in low- Earth orbits, low enough to reach existing ICBM boosters before their engines would burn out, but high enough to improve the likelihood of surviving and to avoid atmospheric drag over a nominal seven-year satellite life. The range of characteristics for proposed SBI systems is summarized in the classified version of this report. It would take a few thousand carrier satellites in nearly polar orbits at several hundred

114 107 km altitude to attack effectively a high percentage of the mid-1990s Soviet ICBM threat. There was a wide range in the number of interceptor rockets proposed by system architects, depending on the degree of redundancy deemed necessary for functional survivability, on the number of interceptors assigned to shoot down Soviet direct-ascent anti-satellite weapons (ASATs), and on the leakage rates accepted for the boost-phase defense. In late 1986, the SDIO and its contractors began to examine options for 1990s deployment which would include constellations of only a few hundred carrier vehicles (CVs) and a few thousand SBIs. This evolved into the phase-one design which, if deployed in the mid to late 1990s, could only attack a modest fraction of the existing Soviet ICBMs in their boost and post-boost phases. Exe-atmospheric Reentry Interceptor System (ERIS). The ERIS would be a ground-based rocket with the range to attack RVs in the late midcourse phase. Existing, but upgraded, radars such as BMEWS, PAVE PAWS, and the PAR radar north of Grand Forks, North Dakota might supply initial track coordinates to ERIS interceptors. 2 (These radars might be the only sensors available for near-term deployments.) Alternatively, new radars or optical sensors would furnish the track data. Upgraded radars would have little discrimination capability (unless the Soviets were to refrain from using penetration aids); moreover, a single high altitude nuclear explosion could degrade or destroy them. Optical sensors might reside on a fleet of space surveillance and tracking system (SSTS) satellites or on ground-based, pop-up probes based at higher latitudes. Such sensors might supply early enough infrared (IR) track data.. The range of planned ground-based radars such as the Terminal Imaging Radar (TIR), which could discriminate RVS from decoys, might be too short to aid ERIS long-range interceptors; the TIR was planned for the lower HEDI endoatmospheric system. A longer-range Ground-based Radar (GBR) system has also been proposed. This system may be capable of supporting ERIS interceptors. Photo Credit: Lockheed Missiles and Space Company How ERIS would work. The ERIS vehicle would be launched from the ground and its sensors would acquire and track a target at long range, ERIS would then maneuver to intercept the target s path, demolishing it on impact.

115 108 Photo Credit: Lockheed Missiles and Space Company ERIS kill vehicle concept. The Integrated Avionics Package (IAP) computer (top left) receives interceptor position data from the on-board Inertial Measurement Unit and target position data from the seeker, or infrared sensor. The seeker acquires and tracks the incoming warhead. The IAP sends guidance commands to the two transverse and two lateral thrusters, which maneuver the vehicle to the impact point. Helium is used to pressurize the fuel tanks and also as a propellant for the attitude control system at the aft bulkhead. The lethality enhancement device would deploy just before impact to provide a larger hit area. to take full advantage of the ERIS fly-out range. 3 If deployed, an airborne optical system (AOS) could give some track data late in midcourse. None of these sensors has been built, although the AirborneOptical Adjunct (AOA), a potential precursor to the AOS airborne system, is under construction and will be test flown in the late 1980s. Anon-board IR homing sensor would guide the interceptor to a collision with the RV in the last few seconds of flight. This homing sensor would derive from the Homing Overlay Experiment (HOE) sensor, which successfully intercepted a simulated Soviet RV over the Pacific on the fourth attempt, in No major improvements in rocket technology would be necessary to deploy an ERISlike system, but cost would be an important factor. The Army s Strategic Defense Command proposes to reduce the size of the launch vehicle in steps. The Army has proposed Whe ERIS, as presently designed, requires a relatively high target position accuracy at hand-off from the sensor. The BSTS would not be adequate for this. Photo Credit: Lockheed Missiles and Space Company ERIS Functional Test Validation (FTV) v. baseline ERIS concept. Sizes of the FTV vehicle and baseline ERIS concepts are compared to a 6-foot-tall man. ERIS is designed as a ground-launched interceptor that would destroy a ballistic missile warhead in space. The FTV vehicle is 33 feet tall, large enough to carry both an observational payload to observe the impact with the warhead and the telemetry to relay information to the ground during the flight tests. The baseline interceptor concept is less than 14 feet tall, more compact because it will not require all the sensors and redundancies that are demanded by flight tests. partly to reduce costs to-test this system with a Functional Technical Validation (FTV) rocket in This missile would have approximately twice the height, 10 times the weight, and twice the burn time of the planned ERIS rocket. The planned ERIS rocket system has a target cost of $1 million to $2 million per intercept in large quantities. Research is proceeding with a view to possible deployment by the mid-1990s. Much development would be necessary to upgrade the experimental HOE kinetic kill vehicle technology for an operational ERIS interceptor. The IR sensors are being radiationhardened. Since the operational sensor could not be maintained at the cryogenically low temperatures required for the HOE experiment, higher operating-temperature sensors are being developed, with cool-down to occur after alert or during rocket flight. High Endo-atmospheric Defense Interceptor (HEDI). The HEDI system would attack RVs that survived earlier defensive layers of ground-based, high-velocity interceptor

116 109 rockets. HEDI would take advantage of the fact that the atmosphere would slow down light-weight decoys more than the heavier RVs. Since it would operate in the atmosphere, HEDI might attack depressed trajectory submarine-launched ballistic missile (SLBM) warheads that would under-fly boost and midcourse defensive layers provided it received adequate warning and sensor data. According to one plan, an AOS would track the RVs initially, after warning from the boostphase surveillance and tracking system (BSTS) and possible designation by SSTS (if available). The AOS would hand target track information off to the ground-based terminal imaging radar (TIR). The TIR would discriminate RVs from decoys both on shape (via doppler imaging) and on their lower deceleration (compared to decoys) upon entering the atmosphere. Interceptors would attack the RVs at altitudes between 12 and 45 km. The HEDI system thus would combine passive optics (IR signature), atmospheric deceleration, and active radar (shape) to distinguish RVs from decoys. The penalty for waiting to accumulate these data on target characteristics would be the need for a large, high-acceleration missile. The HEDI would have to wait long enough to provide good atmospheric discrimination, but not so long that a salvage-fused RV would detonate a nuclear explosion close to the ground. To accelerate rapidly, the HEDI 2-stage missile must weigh about five to six times more than the ERIS missile. The key technology challenge for the HEDI system would be its IR homing sensor. This non-nuclear, hit-to-kill vehicle would have to view the RV for the last few seconds of flight to steer a collision course. 4 But very high acceleration up through the atmosphere would severely heat the sensor window. This heated window would then radiate energy back to the IR sensor, obscuring the RV target. In addition, atmospheric turbulence in front of the window could further distort or deflect the RV 4 The HEDI interceptor would probably include an explosively driven lethality enhancer. image. No sensor has been built before to operate in this environment. The proposed solution is to use a sapphire window bathed with a stream of cold nitrogen gas. A shroud would protect the window until the last few seconds before impact. Since reentry would heat the RV to temperatures above that of the cooled window, detection would be possible. Recent testing gives grounds for optimism in this area. Fabrication of the sapphire windows (currently 12 by 33 cm) would be a major effort for the optics industry. These windows must be cut from crystal boules, which take many weeks to grow. At current production rates, it would take 20 years to make 1,000 windows. Plans are to increase the manufacturing capability significantly. The HEDI sensor suite also uses a Nd:YAG 5 laser for range finding. Building a laser ranger to withstand the high acceleration could be challenging. As with ERIS, plans call for testing a HEDI Functional Technical Validation missile, which is 2 to 3 times larger than the proposed operational vehicle. The proposed specifications of HEDI are found in the classified version of this report. Flexible Light-Weight Agile Experiment (FLAGE). The weapon system expected to evolve from FLAGE research would be the last line of defense, intercepting any RVs which leaked through all the other layers. Its primary mission would be the defense of military targets against short range missiles in a theater war such as in Europe or the Middle East. The FLAGE type of missile would intercept RVs at altitudes up to 15 km. The homing sensor for FLAGE would use an active radar instead of the passive IR sensor proposed for on all other KEW homing projectiles. 5 Nd:YAG is the designation for a common laser used in research and for military laser range-finders. The Nd represents neodymium, the rare element that creates the lasing action, and YAG stands for yttrium-alumin urn-garnet, the glasslike host material that carries the neodymium atoms.

117 110 The FLAGE system was flown six times at the White Sands Missile Range. On June 27, 1986, the FLAGE missile successfully collided with an RV-shaped target drone which was flown into a heavily instrumented flight space. The collision was very close to the planned impact point. Another FLAGE interceptor collided with a Lance missile on May 21, The FLAGE program ended in mid-1987 with the Lance intercept. A more ambitious Extended Range Intercept Technology (ER- INT) program succeeds it. The ERINT interceptors will have longer range and a lethality enhancer. FLAGE was a fire-and-forget missile; no information was transmitted from any external sensor to the missile once it was fired. The ERINT missiles are to receive midcourse guidance from ground-based radars. Six test launches are planned at the White Sands Missile Range. KEW Technology Three types of KEW propulsion have been proposed for SDI: conventional projectiles powered by chemical energy, faster but less well-developed electromagnetic or railgun technology, and nuclear-pumped pellets. All system architects nominated the more mature chemically propelled rockets for near-term BMD deployments. How Chemical Energy KEWs Work. There are three different modes of operation proposed for chemically propelled KEWs: space-based rockets attacking boosters, post-boost vehicles (PBVs), RVs, and direct-ascent ASATs; ground-based rockets attacking RVs in late mid-course outside the atmosphere, and ground-based rockets attacking RVs inside the atmosphere. Two or more rocket stages would accelerate the projectile toward the target. The projectile would be the heart of each system and would entail the most development. The smart projectile for the space-based mission would need some remarkable features. It would be fired at a point in space up to hundreds of seconds before the actual interception. 6 After separation from the last rocket stage, the projectile would have to establish the correct attitude in space to see the target: in general the line-of-sight to the target would not correspond with the projectile flight path. If it had a boresighted sensor that stared straight ahead, then the projectile would have to fly in an attitude at an angle to its flight path to view the target (see figure 5-1). 7 The projectile would have to receive and execute steering instructions via a secure communications channel from the battle manager. Usually just a few seconds before impact, the projectile would need to acquire the target either a bright, burning booster or a much dimmer PBV with an on-board sensor. It would then make final path corrections to effect a collision. Fractions of a second before impact, it might deploy a lethality enhancement device like the spider-web structure used in the Army s Homing Overlay Experiment (HOE) to increase the size of the projectile and therefore its chance of hitting the target. The SBI projectile must have these components: an inertial guidance system, a secure communications system, a divert propulsion system, an attitude control system, a sensor for terminal homing (including vibration isolation), a lethality enhancement device (optional?), and a computer able to translate signals from the sensor into firing commands to the divert propulsion system in fractions of a second. The on-board sensors envisaged by most system architects for more advanced phase-two 6 A computer in the battle management system would estimate the actual interception aim-point in space by projecting the motion or track of the target using the sensor track files. 7 For non-accelerating targets, this look angle would not change, even though the target and the projectile were traveling at different velocities. In this proportional navigation mode, the projectile orientation would be fixed once the sensor was aimed at the target.

118 111 Figure 5.1. Orientation of SDI to RV (SBI) Orientation of the space-based interceptor (SBI) to the reentry vehicle (RV) during the homing phase of the flight. (This drawing shows a sensor bore-sighted with the axis of the SBI, which is common for guided missiles operating in the atmosphere. For space-based interceptors, the sensor could just as well look out the side of the cylindrical projectile.) The SBI sensor would have to be aimed at the RV so that its lineof-sight would not be parallel to the SBI flight path (except for a head-on collision.) For a non-accelerating RV, the angle from the sensor line-of-sight to the SBI flight path would be fixed throughout the flight. Since targets such as ICBM boosters and post-boost vehicles do change acceleration during flight, then this look angle and hence the orientation of the SBI would have to be changed during the SBI flight. SOURCE Office of Technology Assessment, 1988, space-based interceptors may be particularly challenging because they would perform several functions. They would track not only the ICBM during the boost phase, but also the PBV, RVs, and direct-ascent ASAT weapons sent up to destroy the BMD platforms. Each SBI would, ideally, kill all four types of targets. In the boost phase, a short-wave infrared (SWIR) or medium-wave infrared (MWIR) sensor with existing or reasonably extended technology could track a hot missile plume. An SBI would still have to hit the relatively cool missile body rather than the hot exhaust plume. Three approaches have been suggested for detecting the cooler missile body: computer algorithm, separate long-wave infrared (LWIR) sensor, or laser designation. A computer algorithm would steer the SBI ahead of the plume centroid by a prescribed distance that would depend on the look angle of the SBI relative to the booster and on the booster type. Predicting the separation between the plume centroid and the booster body under all conditions might be difficult or even impractical if that separation varied from one booster to the next. A separate LWIR sensor channel might acquire and track the cold booster body. 8 One designer proposed a single detector array, sensitive across the IR band, in combination with a spectral filter. This filter would move mechanically to convert the sensor from MWIR to LWIR capability at the appropriate time. Finally, in some designs a separate laser on the weapon platform or on an SSTS sensor would illuminate the booster. In this case a narrow-band filter on the interceptor s sensor would reject plume radiation, allowing the SBI to home in on laser light reflected from the booster body. In the post-boost and mid-course phases of the attack, the SBI would have to track hot or warm PBVs and cold RVs. Therefore either SBIs would need to have much more sophisticated LWIR sensors, or they would need something like laser designators to enhance the target signature. This laser illumination need not be continuous, except possibly during the last few seconds before impact. But intermittent illumination would place another burden on the battle manager: it would have to keep track of all SBIs in flight and all SBI targets, then instruct the laser designator at the right time to illuminate the right target. An SBI lethality enhancer might, for example, consist of a spring-loaded web which ex- There is also a possibility that an SWIR or MWI R sensor could acquire a cold booster body. At 4.3 p, for exaznple, the atmosphere is opaque due to the C0 2 absorption, and the upper atmosphere at a temperature of 2200 K would be colder than a booster tank at 3000 K. As an SBI approached a booster, the latter would appear to a 4.3 ~m sensor as a large, warm target against the background of the cool upper atmosphere,

119 112 panded to a few meters in diameter or an explosively propelled load of pellets driven radially outward. Weight would limit the practical diameter of expansion. System designers would have to trade off the costs of increased homing accuracy with the weight penalty of increased lethality diameter. Ground-based KEW capabilities would resemble those of space-based interceptors. Exoatmospheric projectiles that intercept the RVs outside the Earth s atmosphere would use LWIR homing sensors to track cold RVs, or they would employ other optical sensors to track laser-illuminated targets. These interceptors would be command-guided to the vicinity of the collision by some combination of ground-based radars, airborne LWIR sensors (AOS) or space-borne LWIR sensors (SSTS, BSTS, or rocket-borne probes). Long-wave infrared homing sensors in the projectile would have to be protected during launch through the atmosphere to prevent damage or overheating. Current Status of Chemically Propelled Rockets. No interceptor rockets with BMDlevel performance have ever been fired from space-based platforms. Operational IR heatseeking interceptor missiles such as the airto-air Sidewinder and the air-to-ground Maverick are fired from aircraft, but both the range and the final velocity of this class of missiles are well below BMD levels. The SDIO s Delta 180 flight test included the collision of two stages from a Delta rocket after the primary task of collecting missile plume data was completed. However, these two stages were not interceptor rockets, were not fired from an orbiting platform, did not have the range nor velocity necessary for BMD, and were highly cooperative, with the target vehicle orienting a four-foot reflector toward the homing vehicle to enhance the signal for the radar homing system. Note that this test used radar homing, whereas all SBI designs call for IR homing or laser-designator homing. This experiment did test the tracking algorithms for an accelerating target, although the target acceleration for this nearly head-on collision was not as stressing as it would be for expected BMD/SBI flight trajectories. 9 Engineers have achieved very good progress in reducing the size and weight of components for the proposed space-based interceptors. They have developed individual ring laser gyroscopes weighing only 85 g as part of an inertial measuring unit. They have reduced the weight of divert propulsion engines about 9 kg to 1.3 kg. Gas pressure regulators to control these motors have been reduced from 1.4 kg to.09 kg each. The smaller attitude control engines and valves have been reduced from 800 g each to 100 g each. Progress has also been made on all other components of a SBI system, although these components have not as yet been integrated into a working prototype SBI system. Ground-based interceptor rockets are one of the best developed BMD technologies. The Spartan and Sprint interceptor missiles were operational for a few months in the mid 1970s. Indeed parts of these missiles have been recommissioned for upcoming tests of SDI groundbased weapons such as the endo-atmospheric HEDI. The production costs for these missiles would have to be reduced substantially to make their use in large strategic defense systems affordable, but no major improvements in rocket technology are needed for groundbased interceptors, other than a 30 percent improvement in speed for the HEDI missile. As discussed in chapter 4, however, major sensor development would be necessary for these interceptors. Key Issues for Chemical Rockets. Chemical rocket development faces four key issues, all related to space-based deployment and all derived from the requirement to design and make very fast SBIs. Constellation Mass. The overriding issue for SBIs is mass. The SBIs must be so fast Previous tests of IR guided projectiles such as the Homing Overlay Experiment against a simulated RV and the F-16 launched ASAT test against a satellite, shot down nonaccelerating targets.

120 113 that a reasonably small number of battle stations could cover the entire Earth. But, for a given payload, faster rockets consume much more fuel-the fuel mass increases roughly exponentially with the desired velocity. The designer must compromise between many battle stations with light rockets or fewer battle stations with heavier rockets. 10 These trade-offs are illustrated in figure 5-2, which assumes a boost-phase-only defense with three hypothetical rocket designs: a stateof-the-art rocket based on current technology; a realistic design based on improvements in rocket technology that seem plausible by the mid-1990s; and an optimistic design that assumes major improvements in all areas of rocket development. The key parameters assumed for SBI rocket technology appear in the classified version of this report. In all cases analyzed, OTA assumed the rockets to be ideal : the mass ratio of each stage is the same, which produces the lightest possible rocket. 11 The first chart in figure 5-2a shows that rocket mass increases exponentially with increasing velocity, limiting practical SBI velocities to the 5 to 8 km/s range for rockets weighing on the order of 100 kg or less. For analytic purposes, OTA has considered constellations of SBIs that would be necessary to intercept virtually 100 percent of postulated numbers of ICBMs. It should be noted that since the system architecture analyses of 1986, SDIO has not seriously considered deploying SBIs that would attempt to intercept anywhere near 100 percent of Soviet ICBMs and PBVS. 12 This OTA analysis is intended only Projectile mass might not be as critical for ground-based as for spacebased KEW projectiles, since there would be no space transportation cost. However, the projectile mass should still be minimized to reduce the over-all rocket size and cost, and to permit higher accelerations and final velocities. llthe ma99 fraction for a rocket stage is defined = the ratio of the propellant mass to the total stage mass (propellant plus rocket structure). The mass fraction does not include the payload mass. For the calculations reported here, an ideal rocket is assumed: it has equal mass ratios for each stage, where mass ratio is defined as the initial stage weight divided by the stage weight after burn-out (both including the payload; it can be shown that the rocket mass is minimized for a given burnout velocity if each stage has the same mass ratio.) AS indicated in chapters 1,2, and 3, SDIO argues that the deterrent utility of defenses far more modest than those needed for assured survival would make them worthwhile. to give a feel for the parameters and trade-offs involved in a system with SBIs. Deployment of a system of state-of-the-art SBIs intended to provide 100 percent coverage of Soviet ICBMs would entail 11.7 million kg of CVs; waiting for the development of the realistic SBI would reduce the mass to orbit by a factor of two. Figure 5-2b shows the number of SBI carrier platforms and figure 5-2c shows the number of SBIs for a 100 percent-boost-phase defense as a function of SBI velocity. The last chart (figure 5-2d) shows the total constellation mass as a function of velocity. The number of CVs was calculated initially to optimize coverage of existing Soviet missile fields: the orbits of the CVs were inclined so that the CVs passed to the north of the missile fields by a distance equal to the SBI fly-out range. 13 Each CV therefore stayed within range of the ICBM fields for a maximum period during each orbit. The optimal number of CVs resulting from this calculation was so low as to endanger system survivability (see ch. 11), calling for up to 100 SBIs per carrier to cover the existing Soviet ICBM threat: such concentrations would provide lucrative targets for the offense s ASATs. To increase survivability, the number of CVs was therefore increased by a factor of 3 for the data in figure 5-2. Some polar orbits were added to cover the SLBM threat from northern waters. The number of SBIs was calculated initially to provide one SBI within range of each of 1,400 Soviet ICBMs sometime during the boost phase. The booster burn time was taken as similar to that of existing Soviet missiles, with a reasonable interval allotted for cloudbreak, initial acquisition, tracking, and weapons launch. One SBI per booster would not do for a robust (approaching 100 percent coverage) boostphase defense. A substantial number of SBIs *The locations of Soviet missile fields are estimated from maps appearing in U.S. Department of Defense, Sow et Mih tary Power, 1987 (Washington, D. C.: Department of Defense, 1987), p. 23. See adaptation of this map in chapter 2 of this OTA report.

121 114 Figure 5-2a. -Space-based Interceptor Mass v. Velocity Figure 5-2c. -Number of Space-Based Interceptors v. Velocity (Inclined orbits + SLBM polar orbits) Velocity (km/see) The SBI mass versus SBI velocity. These data assume 100 /0 coverage of the current Soviet threat of 1,400 ICBMs. It should be noted that the SDIO currently proposes a substantially lower level of coverage for SBIs. Therefore, the absolute numbers in the OTA calculations are not congruent with SDIO plans. Rather, the graphs provided here are intended to show the relationships among the various factors considered. It should also be noted that numerous assumptions underlying the OTA analyses are unstated in this unclassified report,, but are available - in the classified version. SOURCE: Office of Technology Assessment, Velocity ~km/see) The number of space-based interceptors (SBIs) required to provide one SBI within range of each of 1,400 existing Soviet ICBMs before booster burnout. SOURCE: Office of Technology Assessment, Figure 5-2d. -Constellation Mass v. SBI Velocity Figure 5-2b.-Number of Satellites v. SBI Velocity Velocity (km/see) I I I I 4 6 Velocity~km/see) The number of SBI carrier satellites v. SBI velocity. SOURCE: Office of Technology Assessment, 1988 The total constellation mass in orbit (SBIs and carrier vehicles, excluding sensor satellites) v. SBI velocity. The minimum constellation mass for the realistic SBI to be in position to attack all Soviet boosters would be about 5.3 million kg. Faster SBIs would permit fewer carrier vehicles and fewer SBIs, but the extra propellant on faster SBIs would result in a heavier constellation. For reference, the Space Shuttle can lift about 14,000 kg into polar orbit, a 5.3 million kg constellation would require about 380 Shuttle launches, or about 130 launches of the proposed Advanced Launch System (ALS), assuming it could lift 40,000 kg into near-polar orbit at suitable altitudes. SOURCE: Office of Technology Assessment, 1988.

122 115 would fail over the years just due to electronic and other component failures. The number of SBIs in figure 5-2 was increased by a plausible factor to account for this natural peacetime attrition. In addition, during battle, some SBIs would miss their targets, and presumably Soviet defense suppression attacks would eliminate other CVs and draw off other SBIs for self-defense. Given the above assumptions, figure 5-2 represents the SBI constellation for nearly 100 percent coverage of the existing Soviet ICBM fleet in the boost phase, with modest survivability initially provided by substantial SBI redundancy, degrading to no redundant SBIs as natural attrition set in. Note that for each type of rocket there is an optimum velocity that minimizes the total mass that would have to be launched into space; lower velocity increases the number of satellites and SBIs, while higher velocity increases the fuel mass. In OTA s analysis, the minimum mass which would have to be launched into orbit for the realistic rocket is 5.3 million kg (or 11.7 million lb); the mass for a constellation of optimistic SBIs would be 3.4 million kg. The data for figure 5-2 all assume booster burn times similar to those of current Soviet liquid-fueled boosters. Faster-burning rockets would reduce the effective range of SBIs and would therefore increase the needed number of carrier satellites. The same SBI parameters are shown in figures 5-3a and b with an assumption of ICBM booster burn time toward the low end of current times. The minimum constellation mass has increased to 29 million and 16 million kg, respectively, for the realistic and optimistic rocket designs. Several studies of fast-burn boosters concluded that reducing burn-time would impose a mass penalty, so the Soviets would have to off-load RVs (or decoys) to reduce burn time significantly. But these same studies showed that there is no significant mass penalty for burn times as low as 120 s. About percent of the payload would have to be off-loaded for burn times in the 70 to 90 s range. Figure 5-3a. -Number of Projectiles v. SBI Velocity (160 second burn-time) Velocity (km/see) The number of space-based interceptors v. SBI velocity for reduced booster burntime (within currently applied technology). SOURCE: Office of Technology Assessment, Figure 5-3b. -SBI Constellation Mass v. SBI Velocity (160 second burn-time) Velocity (km/see) The total constellation mass (carrier vehicles and SBIs) versus SBI velocity for reduced booster burn times, assuming one SBI within range of each of 1,400 boosters before burnout. SOURCE: Office of Technology Assessment, 1988 If the Soviet Union could reduce the burn time of its missiles below that of any currently deployed ICBMs, then the total SBI constellation mass necessary for boost-phase intercept would increase dramatically. The minimum constellation mass to place one SBI within range of each ICBM during its boost phase is shown in figure 5-4 as a function of

123 116 Figure 5-4.-Total SBI Constellation Mass in Orbit v. Booster Burn Time Realistic rocket Booster bum time (seconds) The effect of Soviet booster burn time on SBI constellation mass. If we consider 40 million kg as a maximum conceivable upper bound on constellation mass (corresponding to 2,800 Shuttle flights or 1,000 launches of the proposed ALS system), then booster times of 120 to 150 seconds would severely degrade a 100%-boost-phase defense with chemically propelled rockets. The ability of smaller constellations of SBIs to achieve lesser goals would be analogously degraded by the faster burn times. All assumptions are the same as for the previous figures, except for the burn-out altitude, which varies with burn-time. SOURCE: Office of Technology Assessment, booster burn time for the three canonical rocket designs. The masses described above for a boostphase-only defense are clearly excessive, particularly for a responsive Soviet threat. Adding other defensive layers would reduce the burden on boost-phase defense. The next layer of defense would attack PBVs, preferably early in their flight before they could unload any RVs. A PBV or bus carrying up to 10 or more RVs would be more difficult to track and hit than a missile. A PBV has propulsion engines that emit some IR energy, but this energy will be about 1,000 times weaker than that from a rocket plume. 14 A PBV is also smaller and less fragile than a booster tank. In short, a PBV is harder to detect and hit with an SBI. However, a PBV is still bigger and brighter than Whe first stage of an ICBM might radiate 1 million W/sr, the second stage 100,000 W/sr, while a PBV may emit only 100 W/sr. On the other hand, the RV radiates only 5 W/sr, so the PBV is a better target than an RV. an RV; sensors might acquire the PBV if its initial trajectory (before its first maneuver) can be estimated by projecting the booster track. The effectiveness of a combined boost and post-boost defense in terms of the percentage of RVs killed is estimated in figure 5-5 for the realistic SBI rocket. The calculation assumes that 1,400 missiles resembling today s large, heavy ICBMs are spread over the existing Soviet missile fields. The net effect of attacking PBVs is to reduce the number of SBIs needed to kill a given number of RVs. For example, to destroy 85 percent of the Soviet RVs carried by ICBMs, a boost-only defense system would require about 26,000 SBIs in orbit. Adding PBV interceptions reduces the number of SBIs needed to about 17,000. A defensive system must meet the expected Soviet threat at the actual time of deployment, not today s threat. For example, the Soviet Union has already tested the mobile, solidfueled SS-24 missile, which can carry 10 war- Figure 5-5. Boost and Post-Boost Kill Effectiveness (1,400 ICBMs v. Realistic" SBIs) o 40 Number of space interceptors (SBIs) (thousands) Percentage of reentry vehicles (RVs) killed as a function of the number of space-based interceptors (SBIs) deployed in space. This calculation assumes a threat of 1,400 ICBMs spread over the Soviet missile fields. The SBIs have a plausible single-shot probability of killing a booster and a slightly smaller chance of killing a PBV; a substantial fraction of the SBIs are used for self-defense (or are not functional at the time of attack). SOURCE: Office of Technology Assessment, 1988,

124 117 heads. There is no reason to doubt that the Soviets could deploy this kind of missile in quantity by the mid-1990s. Such a fleet would particularly stress a space-based defense if deployed at one or a few sites, since more SBIs would be needed in the area of deployment concentration. The effects on the combined boost and postboost defense of clustering 500 shorter-burntime, multiple-warhead missiles at three existing SS-18 sites are shown in figure 5-6a. It would take about 23,000 SBIs to stop 85 percent of these 5,000 warheads. If the assumed 500 ICBMs were concentrated at one site (but still with 10 km separation to prevent pindown by nuclear bursts), then 30,000 SBIs would be needed (see fig. 5-6b). l5 Finally, the Soviets might deploy 200 (or more) current-technology, single-warhead missiles atone site, as shown in figure 5-7. In this case, no reasonable number of SBIs could intercept 85 percent of these 200 extra warheads (50,000 SBIs in orbit would kill 70 percent). Twice as many RVs are destroyed in the postboost period as the boost-phase. Once this concentrated deployment was in place, the defense would have to add about 185 extra SBIs and their associated CVs to achieve a 50 percent probability of destroying each new ICBM deployed. SBI Projectile Mass. The constellation masses shown above assume that the mass of the smart SBI projectile (including lateral divert propulsion, fuel, guidance, sensor, communications, and any lethality enhancer) can be reduced to optimistic levels. Current technology for the various components would result in an SBI with a relatively high mass. Thus mass reduction is essential to achieve the results outlined above; total constellation mass would scale almost directly with the achievable SBI mass. Concentrating 500 missiles atone site would have disadvantages for an offensive attack: timing would be complicated to achieve simultaneous attacks on widely separated U.S. targets, and Soviet planners may be reluctant to place so many of their offensive forces in one area, even if the missiles are separated enough to prevent one U.S. nuclear explosion from destroying more than one Soviet missile. Figure 5-6a. Boost and Post-Boost Kill Effectiveness (500 single.rv ICBMs at three sites) ( Realistic SBIs) -o Number of space-based interceptors (SBIs) (thousands) The percentage of RVs from modestly short-burn ICBMs killed as a function of the number of SBIs deployed in space. This curve corresponds to 500 such ICBMs deployed at 3 existing SS-18 sites. All SBI parameters are the same as in previous figures. SOURCE: Office of Technology Assessment, 1988 Figure 5-6b. Boost and Post-Boost Kill Effectiveness (500 singie.riv ICBMs at one site) Number of space-based interceptors (SBIs) (thousands) This curve assumes that all 500 shorter-burn ICBMs are deployed at one site (but still with 10 km separation to prevent pin-down). All other parameters are the same as figure 5-6a. SOURCE: Office of Technology Assessment, 1988, Rocket Specific Impulse. Similarly, the specific impulse of the rocket propellant would have to be improved from current levels. The specific impulse, expressed in seconds, measures the ability of a rocket propellant to change mass into thrust. It is defined as the ratio of thrust (lb) divided by fuel flow rate (lb/s). I 1

125 118 Figure Boost and Peat-Boost Kill Effectiveness (200 medium-burn-booster ICBMs at one site) ( Realistic SBIs) Number of space-based interceptors (SBIs) (thousands) Percentage of single-warhead ICBM RVs killed as a function of number of SBIs in space. The 200 single-warhead ICBMs are deployed at one site with 10 km separation. All SBI parameters are as in previous figures. SOURCE: Office of Technology Assessment, The specific impulse of current propellants varies from 240 to 270 s for solid fuel and up to 390s at sea level for liquid oxygen and liquid hydrogen fuel. Assuming that BMD weapons would utilize solid fuels for stability and reliability, then the specific impulse for current technology would be limited to the 270-s range. 16 One common solid propellant, hydroxyl-terminated polybutadiene (HTPB) loaded with aluminum, has an impulse in the 260-to-265-s range. This can be increased to 280s by substituting beryllium for the aluminum. Manufacturers of solid propellant say that further improvements are possible. Rocket Mass Fraction. Finally, the mass fraction-the ratio of the fuel mass to the stage total mass (fuel plus structure but excluding payload) would have to be raised to meet SDI objectives. Large mass fractions can be achieved for very big rockets having 95 percent of their mass in fuel. It would be more difficult to reduce the percentage mass of structure and propulsion motor components for very small SBI rockets. IThe SBI divert propulsion system in the final projectile stage would probably use liquid fuel, and some have suggested that the second stage also use liquid fuel. I The mass fraction can be increased by reducing the mass of the rocket shell. New lighteight, strong materials such as carbon graphite fiber reinforced composite materials or judicious use of titanium (for strength) and aluminum (for minimum mass) may permit increased mass fractions for future rockets. How Electromagnetic Launchers (EML) Work. Electromagnetic launchers or railguns use electromagnetic forces instead of direct chemical energy to accelerate projectiles along a pair of rails to very high velocities. The goal is to reach higher projectile velocities than practical rockets can. This would extend the range of KEW, expanding their ability to attack faster-burn boosters before burn-out. Whereas advanced chemically propelled rockets of reasonable mass (say, less than 300 kg) could accelerate projectiles to at most 9 to 10 km/s, future EML launchers might accelerate small projectiles (1 to 2 kg) up to 15 to 25 km/s. SDIO has set a goal of reaching about 15 km/s. In principle, chemical rockets could reach these velocities simply by adding more propellant. The efficiency of converting fuel energy into kinetic energy of the moving projectile decreases with increasing velocity, however: the rocket must accelerate extra fuel mass that is later burned. A projectile on an ideal, staged rocket could be accelerated to 15 km/s, but only 17 percent of the fuel energy would be converted into kinetic energy of the projectile, down from 26 percent efficiency for a 12 km/s projectile. Since a railgun accelerates only the projectile, it could theoretically have higher energy efficiency, which would translate into less mass needed in orbit. In practice, however, a railgun system would not likely weigh less than its chemical rocket counterpart at velocities below about 12 km/s, since railgun system efficiency would probably be on the order of 25 percent at this velocity.17 17ms assumes 50 ~rmnt efficienc y for converting fuel (thermal) energy into electricity, 90 percent efficiency in the pulse forming network, and 55 percent rail efficiency in converting electrical pulses into projectile kinetic energy. The SDIO has a goal of reaching 40 percent overall EML system efficiency, but this would require the development of very high temperature (2,000 to 2,500 K) nuclear reactor driven turbines. The total system mass might still exceed that of a comparable chemical rocket system.

126 119 Therefore a railgun system would have to carry as much or more fuel than its chemical rocket equivalent-in addition to a massive rocketengine generator system, an electrical pulseforming network to produce the proper electrical current pulses, and the rail itself. The conventional railgun (see figure 5-8) contains a moving projectile constrained by two conducting but electrically insulated rails. A large energy source drives electrical current down one rail, through the back end of the moving projectile, and back through the other rail. This closed circuit of current forms a strong magnetic field, and this field reacts with the current flowing through the projectile to produce a constant outward force. The projectile therefore experiences constant acceleration as it passes down the rail. The final velocity of the projectile is proportional to the current in the rail and the square root of the rail length; it is inversely propor- Figure 5-8.-Schematic of an Electromagnetic Launcher (EML) or Railgun Schematic of an electromagnetic launcher (EML) or Railgun. In operation, a strong pulse of electrical current forms a circuit with the conducting rails and the projectile. This current loop generates a magnetic field. The interaction of this field with the current passing through the moving projectile produces a constant outward force on the projectile, accelerating it to high velocities. SOURCE Office of Technology Assessment, tional to the square root of the projectile mass. High velocity calls for very high currents (millions of amperes), long rails (hundreds of m), and very light projectiles (1 to 2 kg). For the BMD mission, the projectile must be smart. That is, it must have all of the components of the chemically propelled SBIs: a sensor, inertial guidance, communications, divert propulsion, a computer, and possibly a lethality enhancement device. The EML projectile must be lighter, and it must withstand accelerations hundreds of thousands times greater than gravity, compared to 10 to 20 g s for chemically propelled SBIs. Researchers at Sandia National Laboratory have proposed another type of EML launcher which would employ a series of coils to propel the projectile. Their reconnection gun would avoid passing a large current through the projectile, eliminating the arcs and sparks of the conventional railgun. The term reconnection derives from the action of the moving projectile: it interrupts the magnetic fields of adjacent coils, and then these fields reconnect behind the projectile, accelerating it in the process. Current Status of EMLs. Several commercial and government laboratories have built and tested experimental railguns over the last few decades. These railguns have fired very small plastic projectiles weighing from 1 to 2,500 g, accelerating them to speeds from 2 to 11 km/s. In general, only the very light projectiles reached the 10 km/s speeds. One figure of merit, or index, for railgun performance is the kinetic energy supplied to the projectile. For BMD applications, SDIO originally set a goal of a 4 kg projectile accelerated to 25 km/s, which would have acquired 1,250 MJ of energy. SDIO officials now state that their goal is a 1 kg projectile at 15 km/s, which would acquire 113 MJ of kinetic energy. The highest kinetic energy achieved to date was 2.8 MJ (317 g accelerated to 4.2 km/s), or about 50 to 400 times less than BMD levels. Finally, there have been no experiments with actual smart projectiles. All projectiles have been inert plastic solids. Some (non-operating)

127 120 Table 5-1. Key Issues for Electromagnetic Launchers (EML) Low-mass (2 kg or less), high acceleration (several hundred thousand g) projectile development. High repetition rate rails (several shots per second for hundreds of seconds). High repetition rate switches with high current (several million A versus 750,000 A) Pulse power conditioners (500 MJ, 5 to 20 ms pulses versus 10 MJ, 100 ms pulses) Efficiency Mass Heat dissipation SOURCE: Office of Technology Assessment, Photo credit: Contractor photo released by the U.S. Department of Defense Electromagnetic launcher. This experimental electromagnetic launcher at Maxwell Laboratories, Inc., San Diego, CA, became operational late in electronic components, including focal plane arrays, have been carried on these plastic bullets to check for mechanical damage. Results have been encouraging. Key Issues for EMLs. Much more research must precede an estimate of the potential of EML technology for any BMD application. The key issues are summarized in table 5-1. There is uncertainty at this time whether all these issues can be favorably resolved. EML Projectile. If based on current technology for sensors, inertial guidance, communications, and divert propulsion systems, the lightest smart projectile would weigh over 10 kg. The total mass must shrink by at least a factor of 5, and the projectile must withstand over 100,000 g s of acceleration. If the projectile could only tolerate 100,000 g s, then the railgun would have to be 112 m long to impart a 15 km/s velocity to the projectile. Higher acceleration tolerance would allow shorter railguns. (200,000 g s would allow a 56-m long gun, etc.) The SDIO has consolidated the development of light-weight projectiles for all kinetic energy programs into the Light-weight Exo-atmospheric Projectile (LEAP) program. Although researchers first saw a need for light-weight projectiles for railguns, the primary initial users of LEAP technology are to be the chemical rocket KEW programs (SBI, ERIS, HEDI). The phase-one LEAP projectile would weigh about 5 kg according to current designs (see figure 5-9), if all component developments met their goals. This projectile would weigh too much for any railgun, and it will therefore not be tested at high acceleration. This technology might evolve into a 2-kg projectile by the early 1990s. In any case, there are no plans now to build a gun big enough to test even the phase-two 2 kilogram projectile. High Repetition Rate. A railgun would have to fire frequently during an attack, engaging several targets per second. The penalty for low repetition rates would be additional railguns in the space-based constellation to cover

128 121 Figure 5-9. Lightweight Homing Projectile LEAP PHASE 1 LEAP PHASE 2 WEIGHT S LEAP PHASE 1 PAYLOAD (1 600) SEEKER IMU INTEGRATED PROCESSOR COMMAND RECEIVER POWER SYSTEM STRUCTURE & MISC PROPULSION (3,059.0 VALVES & NOZZLES CASE & INERTS PROPELLANT VALVE DRIVERS STATEMENTS LEAP PHASE 2 PAYLOAD (670) SEEKER IMU 70.0 INTEGRATED PROCESSOR COMMAND RECEIVER 25.0 POWER SYSTEM STRUCTURE & MISC PROPULSION ( VALVES & NOZZLES CASE & INERTS PROPELLANT VALVE DRIVERS 45.0 PROJECTILE TOTAL 4659.( PROJECTILE TOTAL 1995,0 Illustration of planned projectiles for the Lightweight Exoatmospheric Projectile (LEAP) program. This program is developing projectile technology for both the rocket propelled and electromagnetic launcher (railgun) programs. However, the phase 1 projectile at 5 kg and even the more conceptual phase 2 projectile with a mass projection of 2 kg are too heavy for any existing or planned railguns. There are no plans to test either projectile at the 100,000s of g acceleration necessary for rail gun operation. These projectiles will benefit the SBI, ERIS, and HEDI programs. SOURCE: Office of Technology Assessment, the threat. Most railguns to date have been fired just once: the rails eroded and had to be replaced after one projectile. Newer systems can fire ten shots per day, and at least one experiment has fired a burst of pellets at a rate of 10/s. Researchers at the University of Texas plan to fire a burst of ten projectiles in 1/6 of a second, or a rate of 60/s. Key issues for high repetition-rate guns are rail erosion, 18 heat management, and high repetition-rate switches to handle the millionampere current levels several times per second. Conventional high repetition-rate switches can *New rail designs have shown promise of minimum erosion in laboratory tests; it remains to be proven that rails would survive at weapons-level speeds and repetition rates.

129 122 handle up to 500 A today, although one special variable resistance switch tested by the Army carried 750,000 A. An Air Force test successfully switched 800,000 A, limited only by the power supply used. EML systems would have to switch 1 to 5 million A. EML Power. An EML would consume high average electrical power and very high peak power during each projectile shot. Consider first the average power requirements: a 1 kg projectile would acquire 112 MJ of kinetic energy if accelerated to 15 km/s. Assuming 40 percent efficiency and 5 shots per second, then the EML electrical system would have to deliver 280 MJ of energy per shot or 1.4 GW of average power during an attack which might last for several hundred seconds. For comparison, a modern nuclear fission power plant delivers 1 to 2 GW of continuous power. The SP-100 nuclear power system being discussed for possible space application would produce only 100 to 300 kw of power. The only apparent near-term potential solution to providing 2.5 GW of power for hundreds of seconds would be to use something like the Space Shuttle Main Engine (SSME) coupled to a turbogenerator. Assuming 50 percent electrical conversion efficiency, then one could convert the SSME 10 GW of flow energy into 5 GW of average electrical power while the engines were burning. High average power would not suffice. The electrical energy would have to be further concentrated in time to supply very short bursts of current to the railgun. For example, a 112- m long railgun with 100,000-g acceleration would propel a projectile down its length in about 15 milliseconds (ins) to a final velocity of 15 km/s. The peak power during the shot would be 50 GW. l9 And the EML system designer would like to shorten the 112-m railgun length and increase acceleration, which would mean further shortening the pulse length and increasing peak power. Several techniques are under consideration to convert the average power from something like the SSME into short pulses. One laboratory approach is the homopolar generator: this device stores current in a rotating machine much like an electrical generator and then switches it out in one large pulse. Existing homopolar generators can supply up to 10 MJ in about 100 ms; therefore, energy storage capacity must increase by a factor of 50 and the pulse length shorten by a factor of 5 to 20. EML Mass to Orbit. The mass of an EML system based on today s technology would be excessive. A homopolar generator to supply 280 MJ per pulse would weigh 70 tonnes alone. 20 The rails would have to be long to limit acceleration on sensitive smart projectiles, which would have to be very strong (massive) to resist the outward forces from the high rail currents. The platform would have to include an SSME-type burst power generator, a thermal management system to dispose of the energy deposited in the rails, divert propulsion to steer the railgun toward each target, and the usual satellite communications and control functions. Given the early stage of EML research, estimates of total platform mass could be in error by a factor of 10. At this time, a total mass of about 100 tonnes would seem likely, meaning that each EML would have to be launched in several parts, even if the United States developed an Advanced Launch System (ALS) that could carry about 40 tonnes maximum per flight to high inclination orbits. It is conceivable that, in the farther term, superconductive electrical circuits could significantly reduce the mass of an EML. Lighter compulsators (see below) might also reduce EML mass. Nuclear-Driven Particles. A nuclear explosion is a potent source of peak power and energy. If even a small fraction of the energy in a nuclear explosion could be converted into kinetic energy of moving particles, then an extremely powerful nuclear shotgun could be imlgfor a fi~e of referen~, consider that the total power av~able from the U.S. power grid is several hundred gigawatts. 20Assuming today s energy density for homopolar generators of 4 kj/kg.

130 123 agined. These particles could be used for interactive discrimination as described above, since the particles would slow down light decoys more than heavy RVs With more power, nuclear-driven particles could conceivably destroy targets. This concept is discussed in more detail in the classified version of this report. Directed-Energy Weapons Directed-energy weapons (DEW) offer the promise of nearly instantaneous destruction of targets hundreds or thousands of km away. While a KEW system would have to predict target positions several minutes in the future and wait for a high speed projectile to reach the intended target, the DEW could in principle fire, observe a kill, and even order a repeat attack in less than a second. DEW Systems Although no DEW are planned for phaseone BMD deployment, both ground-based and space-based DEW systems are possible in the next century.21 Candidate DEW systems include: free electron lasers (FEL) (ground-based or space-based), chemical lasers (space-based), excimer lasers (ground-based), x-ray laser (pop-up or space-based), or neutral particle beam (space-based). The FEL is the primary SDIO candidate for ground-based deployment (with the excimer laser as a back-up). The hydrogen-fluoride (HF) laser and the neutral particle beam weapon are the primary candidates for space-ased DEWS, although a space-based FEL or other chemical laser concepts might also be possible. Ground-Based Free Electron Laser (GBFEL). A GBFEL system would include several ground-based lasers, rubber mirror beam directors to correct for atmospheric distortions and to direct the beams to several relay mirrors in high-earth orbit, and tens to hundreds ZISDICI a9wrt9 that some versions of DE W could be deployed late in this century. It is examining designs for entry level systems with limited capabilities. of battle-mirrors in lower Earth orbit to focus the beams on target. It would take several laser sites to assure clear weather at one site all the time. Several lasers per site would provide enough beams for the battle. Ideally these lasers should beat high altitudes to avoid most of the weather and atmospheric turbulence. But the FEL, as currently envisioned, requires very long ground path lengths for beam expansion and large quantities of power. The logical location for relay mirrors would be geosynchronous orbit, so that the groundbased beam director would have a relatively fixed aim point. The effects of thermal blooming 22 may best be avoided, however, by placing the relay mirrors in lower orbit: the motion of the laser beam through the upper atmosphere as it follows the moving relay mirror would spread the thermal energy over a large area. 23 Adaptive optics would correct for atmospheric turbulence. The optical system would sense turbulence in real time and continuously change the shape of the beam-director mirror to cancel wave-front errors introduced by the air. A beacon would be placed just far enough in front of the relay satellite that the satellite would move to the position occupied by the beacon in the time it took for light to travel to the ground and back. A sensor on the ground would detect the distortions in the test beam of light from the beacon, then feed the results to the rubber mirror actuators. With its wave front so adjusted, the laser beam would pass through the air relatively undistorted. z Thermal blooming occurs when a high-power laser beam passes through the atmosphere, heating the air which disturbs the transmission of subsequent beam energy. See the section below on key DEW issues for details. 23 For exmple, a 1O-m dimekr laser beam which tracked a relay mirror at 1,000 km altitude would pass through a clean, unheated patch of air at 10 km altitude after 140 ms. If thermal blooming resulted from relatively long-term heating over a few seconds, then scanning across the sky could ameliorate its effects. While beam energy at altitudes below 10 km would take longer than 140 ms to move to unheated patches of the atmosphere, lower altitude blooming could be more readily corrected by the atmospheric turbulence compensation systems proposed for ground based lasers: atmospheric compensation works best for thin lens aberrations close to the laser beam adaptive mirror on the ground.

131 124 This concept is discussed further below, under the heading of Key DEW Issues. Table 5-2 compares the characteristics of current research FELs with those needed for BMD operations, as derived horn elementary considerations in the American Physical Society study.24 The key figure of merit is beam brightness, defined as the average laser output power (watts) divided by the square of the beam s angular divergence. Brightness is a measure of the ability of the laser beam to concentrate energy on the target (see figure 5-10). Another important figure of merit is the retarget time the time needed to switch from one target to another. ~iamerican Physical Society, op. cit., footnote 1, chapters 3 and 5. Existing FELs operate in a pulsed mode: the energy is bunched into very short segments, as illustrated in figure 5-11 for the radio frequency linear accelerator (RF linac) and for the induction linear accelerator, two types of accelerators proposed for the FEL. The power at the peak of each pulse is much higher than the average power. In the proposed induction linac FEL, peak power might exceed average power by 60,000 times. But it is the average power that primarily determines weapons effestiveness. 26 The RF linac experiments to date have produced 10 MW of peak power at 10 µm wave Short pulses of energy may foster coupling of energy into a target, however, so the average power required from a pulsed laser could, in principle, be less than the average power of a continuous wave (CW) laser. This will be the subject of further SDI research. Table 5-2. Characteristics of a Ground-Based FEL Weapons System Current status Operational requirements against a fully responsive Soviet threat a Free Electron Laser RF Induction Number of laser sites Wavelength µm) ,800.8 to 1.3 Average power (MW) to 1,000 Peak power (MW) ,000 b Beam diameter (m) (4) C (4) 10 to 30 Brightness (W/sr) x x 10 6 several x1022 Peak brightness (W/sr) x x 10 d 13 Beam director: Diameter (m) (4) 10 Number of actuators (10 3 ) 10 3 to 10 4 Frequency response (Hz) hundreds Relay mirrors: Number of mirrors ? Diameter (m) or more Altitude (km) tens of thousands Steering rate (retargets/s) o BattIe-mirrors: Number of mirrors: Diameter (m) (4) 10 Altitude (km) ,000-4,000 Steering rate (retargets/s) operational requirements are taken from American Physical Society, Science and Technology of Dkected-Energy Weapons: Report to the American Physical Society of the Study Group, April 19S7. SDIO dlsagreea with some of the numbers, but their disagreements are classified and may be found in the classified version of this report. Further, SDIO has identified BMD missions other than dealing with a fully responsive Soviet threat. An entry-level syatem (with a brightness on the order of 10*0), might be developed earlier than the one with the above characteristics and would have less stressing requirements. bsagment5 of ~ S.meter ~tive minor have baen built, and a A-meter, T.segmmt mirror is under construction. Parentheses in this table indicate that the mirror t=hnology exists, but the mirrors have not yet been integrated with the laser. CA weapons Systam would require the average ~wer levels listed above. The FEL is a pulsed laser the powar of each pulsa is much higher than the average power when the pulses are both on and off. Depending on how targets and pulses Intaract, these short pulses might be lethal even with lower average power. dpeak brightness, like peak power, is not the relevant measure Of weapons lethatity. ethe American physl~ai s~lety, op. cit., footnote a, estimated that brightnesses on the order of 10 Wlsr might be necessary Io counter a responsive threat. A lo-meter diameter mirror would be requirad for the lower power (1 OOMW) FEL module to raach 10 * WLsr brightness. The more probable approach would be to combine the beams from ten lo-meter mirrors in a coherent array. SOURCE: Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organization, 19S7 and 19SS.

132 125 Figure Illustration of the Relationships Between Laser Parameters and Power Density Projected on a Target Figure FEL Waveforms Radio frequency LINAC FEL waveform Laser D Laser output power = P (watts) Beam diameter = D (meters) For a diffraction-limited beam, Duty cycle: Much lower than radio frequency LINAC Brightness = B wavelength) Power density on target = I (watts/square cm) R 2 Illustration of the relationships between laser parameters and power density projected on a target. The key figure of merit for any laser is its brightness. Brightness measures the ability of the laser to concentrate power on a distant target. High brightness requires high laser power and low angular divergence. Low angular divergence in turn requires short wavelength and a large beam diameter. The power density on target is equal to the laser brightness divided by the square of the distance to that target. SOURCE: Office of Technology Assessment, length, but only 6 kw of average power which would translate into a brightness 100,000,000 times less than the level needed for a BMD weapon against hardened Soviet boosters. 26 This 6-kW average power was averaged over a 100-microsecond long macropulse in a given second. ZeThi9 brightne99 calculation assumes that the beam would be expanded to fill a stateof-theart 4-m diameter mirror and was diffraction-limited. Existing laser waveforms from the radio frequency linear accelerator (RF Iinac) free electron laser (FEL) and the induction linear accelerator FEL. The laser light is emitted in very short pulses. The peak power during these short pulses would have to be extremely high to transmit high average power to the targets. This peak intensity, particularly for the induction Iinac FEL, would stress mirror coatings and could induce other nonlinear losses such as Raman scattering in the atmosphere. Therefore, a weapon-grade induction-linac FEL would have to have higher repetition rates, perhaps on the order of 10 kilohertz. SOURCE: Office of Technology Assessment, It should be noted, however, that these experiments were not designed for maximum average power. Low repetition rates were used primariliy for economic reasons. SDIO scientists say that scaling up the number of macropulses from 1/s to 5,000/s is not a serious problem. If correct, this would mean that 30-MW average power could be produced with technology not radically different from today s. In addition, aground-based weapon would use a wavelength an order of magnitude smaller. The brightness scales as the inverse of the wavelength squared. For a given mirror diameter, then, if a similar power output could be produced at a smaller wavelength, and the high repetition rate were achieved, the brightness would only need to be increased by a factor of about 200 for 30 MW at 1 µm. Accomplish-

133 126 The laser should operate at short wavelength (to keep the mirror sizes small) and should be energy efficient (to reduce the weight of fuel needed in orbit). Although its wavelength band (near 2.8 µm) is rather long, the hydrogen fluoride (HF) laser is the most mature and most efficient laser available today. Table 5-3 compares the characteristics of a potential high performance HF laser BMD system with the current mid-infrared chemical laser (MIRACL) (using deuterium fluoride, or DF) operating at the White Sands Missile Range in New Mexico. 28 Photo credit: U.S. Department of Defense Model of ALPHA experimental chemical laser. This experimental chemical laser and its large vacuum chamber have been constructed by TRW at a test site near San Juan Capistrano, CA. The cylindrical configuration of the laser design may be most suitable for basing in space. ing both modifications would entail significant development work. Space-based Chemical Lasers. Placing highpower lasers directly on satellites would eliminate the needs for atmospheric compensation, redundant lasers to avoid inclement weather, and relay mirrors in high orbits; it would also reduce beam brightness requirements by a factor of 4 to 10 (depending on the wavelength and atmospheric factors) since the atmosphere would not attenuate the beam. 27 These advantages are offset by the engineering challenge of operating many tens or hundreds of lasers autonomously in space and by the possible higher vulnerability of lasers relative to battlemirrors. 27 0ne defense contractor estimated that a space-based chemical laser system, including space transportation, would cost about 10 times less than the proposed ground-based free electron laser weapon system. DEW Technology How DEW Work. Directed energy weapons would change stationary, stored energy from a primary fuel source into a traveling beam of energy that could be directed and focused on a target. Several stages of energy conversion may be necessary. The challenge is to build an affordable, survivable, and reliable machine that can generate the necessary beam of energy. Lasers can be driven by electrical energy, chemical energy, or nuclear energy. Free Electron Lasers. Through 1987, the SDIO chose the FEL research program to receive the most DEW emphasis (recently, SDIO has returned to favoring research in spacebased chemical lasers). The FEL uses a relativistic 29 electron beam from an accelerator to amplify a light beam in a vacuum. The key advantage of the FEL is the lack of a physical gain medium: all other lasers amplify light in a solid, liquid, or gas. This gain medium must be stimulated with energy to produce an excited population inversion of atoms or molecules. The fundamental limitation with these lasers is the need to remove waste heat before it affects the optical transparency of the medium. The FEL achieves its gain while pass- 28 The SDIO is also considering lower-performance, entry level, space-based chemical lasers for more limited BMD missions. 29 A beam of particles is deemed relativistic when it is accelerated to speeds comparable to a fraction of the speed of light and acquires so much energy that its mass begins to increase measurably relative to its rest mass.

134 127 Table 5-3. Characteristics of an HF Laser Weapons System Current status Estimated operational requirements a Number of laser satellites Altitude (km) ,000 Beam diameter (m) b 10 Power (MW) greater than 1 hundreds (single beam) Brightness (W/sr) several x 10 17C several x Phased array alternative: Number of beams Beam diameter (m) Total power (MW): (14 MW per beam) athese numbers derived from first principles, and from American Physical Society, SCienCe and Technology of Directed-Energy Weapons. Report of ttre American Physical Society Study Group, April 1987, which contains estimates of booster hardness for a fully responsive threat The SDIO neither confirms nor denies these estimates. Current SDIO estimates may be found In the classified version of this report, In addition, SDIO has identified earlier entry-level systems with less stressing mls. sions and less stressing requirements with brightnesses on the order of 1010 WLsr. bth e LAMP mirror, not yet integrated with a high-power laser, has a diameter of 4 m. CAS~Unling perfect beam quality for a multi-megawatt system with the characteristics of the MIRACL laser SOURCE Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organ i- zat!on, 1987 and 19fM3 ing through an electron beam plasma, so much of the waste heat exits the active region along with the electron beam at nearly the speed of light. Two types of electron beam accelerator are currently under investigation in the SDIO program: the radio frequency linear accelerator (RF linac) and the induction linac. 30 In the RF linac, electrical energy from the primary source is fed to radio-frequency generators that produce an RF field inside the accelerator cavity. This field in turn accelerates low energy electrons emitted by a special source in the front end of the accelerator. The accelerator raises this electron beam to higher and higher energy levels (and hence higher velocity) and they eventually reach speeds approaching that of light. Simultaneously, the electrons bunch into small packets in space, corresponding to the peaks of the RF wave. This relativistic beam of electron packets is inserted into an optical cavity. There the beam passes through aperiodic magnetic field (called a wiggler magnet) that causes the electrons Other types of accelerators are possible for a free electron laser, such as the electrostatic accelerator FEL under investigation at the University of California at Santa Barbara, but the RF linac and induction linac have been singled out as the primary candidates for initial SDI experiments. to oscillate in space perpendicular to the beam axis. As a result of this transverse motion, weak light waves called synchrotrons radiation are generated. Some of this light travels along with the electron packets through the wiggler magnets. Under carefully controlled conditions, the electron beam gives up some of its energy to the light beam. The light beam is then reflected by mirrors at the end of the optical cavity and returns to the wiggler magnet synchronously with the next batch of electrons. The light beam picks up more energy from each pass, and eventually reaches high power levels. This type of FEL is an optical oscillator : it produces its own coherent light beam starting from the spontaneous emission from the synchrotrons radiation. As more energy is extracted from the electron beam, the electrons slow down. These slower electrons are then no longer synchronized with the light wave and the periodic magnet, so the optical gain (amplification) saturates. To increase extraction efficiency, the wiggler magnet is tapered : the spacing of the magnets or the magnetic field strength is varied so that the electrons continue in phase with the light wave and continue to amplif y the beam as energy is extracted. For high-power weapon applications, the power from an oscillator might be too weak:

135 128 the limit for an RF linac FEL oscillator is near 20 MW. In this case additional single-pass amplifiers can boost the beam energy. This system is called a master oscillator power amplifier (MOPA) laser. In the second type of FEL, the induction linac, large electrical coils accelerate narrow pulses of electrons. The high energy electrons interact with an optical beam as in the RF linac FEL, but the optical beam as currently planned would be too intense to reflect off mirrors and recirculate to pickup energy in multiple passes as in the RF oscillator. Rather, all of the energy transfer from the electron beam to the optical beam would occur on a single pass. This would entail very high gain, which demands very high density electron beams and very intense laser light coming into the amplifier. The induction linac FEL therefore depends on an auxiliary laser to initiate the optical gain process; this limits the tunability of the induction linac FEL to the wavelengths of existing conventional lasers of moderately high power. The process of converting electron energy into light energy can theoretically approach 100 percent efficiency, although it may take very expensive, heavy, and fragile equipment. 3l Nevertheless, the FEL could achieve very high power levels, and, unlike other lasers, the RF linac FEL can be tuned to different wave lengths by changing the physical spacing or field strength of the wiggler magnets or the energy of the electron beam. 92 Tunability is desirable for ground-based lasers, which must avoid atmospheric absorption bands (wavelengths of light absorbed by the air) if they are to reach into space. Chemical (EIF) Lasers. The HF laser derives its primary energy from a chemical reaction: deuterium and nitrogen trifluoride ~ltot~ sy9~m efficienw would probably be about 20 Per@nt - 25 percent at best, assuming areasonably optimistic 50 percent- 60 percent efficiency to convert chemical to electrical energy using a rocket-driven turbine, and 40 percent efficiency to generate RF power. The wavelength of the FEL is proportional to the wiggler magnet spacing and inversely proportional to the square of the electron beam energy. Higher beam energies are necessary for the short wavelengths needed for BMD. gases react in a device resembling a rocket engine. Hydrogen gas mixes with the combustion products. Chemical energy raises the resulting HF molecules to an excited state, from, which they relax later by each emitting a photon of light energy in one of several wavelength lines near 2.8 µm in the MWIR. A pair of opposing mirrors causes an intense beam of IR energy to build up as each pass through the excited HF gas causes more photons to radiate instep with the previously generated light wave. 33 Some additional electrical energy runs pumps and control circuits. Excimer Laser. In an excimer 34 laser, electrical energy, usually in the form of an electron beam, excites a rare gas halide 35 such as krypton fluoride or xenon chloride. 36 These gases then emit in the ultraviolet (UV) region of the spectrum, with wavelengths in the range from.2 to.36pm. This very short wavelength permits smaller optical elements for a given brightness. However, the optical finish on those UV optics would have to be of proportionately higher quality. Ultraviolet light is also desirable for space applications, since its high energy generally causes more damage to the surfaces of targets than does that of longer-wavelength visible or IR light. One drawback is that internal mirrors resistant to UV radiation damage are more difficult to make. Another is that UV cannot readily penetrate the atmosphere. These obstacles, combined with their relative immaturity and low efficiency, have relegated high power excimers to a back-up role to the FEL for the ground-based BMD laser. This process of repeat43d radiation in step is called stimulated emission : the traveling wave of light stimulates the excited molecule to radiate with the same phase and direction as the stimulating energy. The resulting beam of light is coherent : it can be focused to a very small spot. The term laser is derived from the phrase Light Amplification by Stimtiatsd Electromagnetic Radiation. ~iexcimer is sho~ for exci~ state dimer ; the excitation of these rare gas halides produces molecules that only exist in the excited state, unlike other lasing media which decay to a ground state after emitting a photon of light. S5A h&de is a compound of two elements, one of which is a halogen: fluorine, chlorine, iodine, or bromine. Krypton fluoride produces a wavelength too short to penetrate the atmosphere; for ground-based applications, xenon chloride would be of interest.

136 129 Passing a laser beam through a Raman gas cell can improve its quality. This cell, typically filled with hydrogen gas, can simultaneously shift the laser frequency to longer wavelengths (for better atmospheric propagation), combine several beams, lengthen the pulse (to avoid high peak power), and smooth out spatial variations in the incoming beams. A low-power, high quality seed beam is injected into the Raman cell at the desired frequency. One or more pump beams from excimer lasers supply most of the power. In the gas cell, Raman scattering transfers energy from the pump beams to the seed. This process has been demonstrated in the laboratory with efficiencies up to 80 percent. X-ray Laser. A nuclear explosion generates the beam of an x-ray laser weapon. Since this type of laser self-destructs, it would have to generate multiple beams to destroy multiple targets at once. It has been proposed that x- ray lasers would be based in the pop-up mode; their launch rockets would wait near the Soviet land mass and fire only after a full-scale ICBM launch had been detected. Since the x- rays could not penetrate deeply into the atmosphere unless self-focused, the earliest application for the x-ray laser would likely be as an ASAT weapon. Neutral Particle Beam (NPB) Weapon. The NPB weapon, like a free electron laser, would use a particle accelerator (see figure 5-12). This accelerator, similar to those employed in high energy physics experiments, would move charged hydrogen (or deuterium or tritium) ions to high velocities. Magnetic steering coils would aim the beam of ions toward a target. As the beam left the device, a screen would strip the extra electrons off the ions, resulting in a neutral or uncharged beam of atoms. 37 Unlike laser beams, which deposit their energy on the surface of the target, a neutral particle beam would penetrate most targets, causing internal damage. For example, a 100- MeV particle beam would penetrate up to 4 37 A charged beam could not be aimed reliably, since it would be deflected by the Earth s erratic magnetic field, so the beam must be uncharged or neutral. Figure Schematic of a Neutral Particle Beam Weapon LH2/Loxturbine Turbogenerator I Liquid oxygen Liquid hydrogen Beam expander magnets Schematic of a neutral particle beam weapon. Primary power might be generated by firing a rocket engine, similar to the Shuttle main engine, coupled to an electrical generator. Alternately, the hydrogen and oxygen could be combined in a fuel cell to produce electricity. The resulting electrical current would drive the accelerator that would produce a beam of negatively charged hydrogen ions. This negatively charged beam would be expanded and directed toward the target by magnets. Just before leaving the device, the extra electron on each hydrogen ion would be stripped off, leaving a neutral particle beam that could travel unperturbed through the earth s erratic magnetic field. SOURCE: Office of Technology Assessment, 1988 cm into solid aluminum and a 200-MeV beam would deposit energy 13 cm deep. 38 These penetrating particles could damage sensitive circuits, trigger the chemical high explosives in nuclear warheads, and-at high enough incident energy levels melt metal components. Shielding against neutral particle beams would be difficult; imposing a large weight penalty. As mentioned in chapter 4, the NPB may be usable first as an interactive discriminator. The beam of energetic hydrogen atoms would dislodge neutrons from massive RVs (the dis- Y3ee W. Barkas and M. Berger, Tables ofenergy Losses and Ranges of Heavy Charged Particles, (Washington, DC: NASA, 1964).

137 130 criminator NPB would presumably dwell on each RV and decoy for too short a time to damage the RV). Separate satellites with neutron detectors would determine which targets were RVs and which were light-weight decoys. The NPB technology development would be the same for the weapon and the interactive discrimination programs, giving it multi-mission capability. Current Status of DEW. Directed energy weapons are at various stages of development as discussed below, but none could be considered ready for full-scale engineering development or deployment in the next decade. 39 The characteristics of three potential DEW systems are summarized in table 5-4. A key figure of merit is the brightness of the beam. Precisely what brightness would destroy different targets is still under investigation: the SD I research program is measuring target lethality for different wavelengths and for different classes of targets. The brightness levels of table 5-4 are derived from physical first principles and assume that the Soviets could convert their missiles to hardened, solid-fueled boosters by the time DEWS could be deployed. 40 WSDIO has recently been considering entry-level options that it currently considers feasible for phase-two deployment. 40SD10 is considering entry level DEWS that would have much lower brightness and might be effective against today s more vulnerable boosters. A synergistic mix of KEW-DEW boost-phase intercept capability and DEW discrimination is being considered by SDIO as possible parts of a phastwo system. The Accelerator Test Stand (ATS) neutral particle beam experimental accelerator at Los Alamos National Laboratory is the weapon candidate closest to lethal operating conditions: its brightness would need to rise by about a factor of 10,000 to assure destruction of electronics inside an RV at typical battle ranges (thousands of km). However, in this kill mode, it maybe hard to determine whether the electronics actually had been destroyed. Another factor of 10 to 100 might be needed to produce visible structural damage. The MIRACL DF chemical laser operating at White Sands has greater than 1 megawatt output power, but its relatively long wavelength, the challenge of unattended space operation, and the uncertainty of scaling this laser to the power levels necessary for ballistic missile defense would make a deployment decision now premature. The brightness of an HF or DF laser would have to be increased by a factor of 10,000 to 100,000 over current levels to be useful against responsively hardened Soviet boosters. However, an entry-level system that might be useful against current boosters would entail an increase in brightness of only several hundred to several thousand times. To test some aspects of a space-based HF laser, TRW is installing its Alpha laser in a large space-simulation chamber near San Juan Capistrano, California. The Alpha laser uses a cylindrical geometry (MIRACL uses lin- Table 5.4. Characteristics of Directed Energy Weapons Against a Fully Responsive Soviet Threat a FEL ground-based HF space-based NPB space-based Primary energy source Electric Chemical Electric Wavelength or energy µm 2.7 µm MeV Required brightness (W/sr)....Several X Several x Several x (for electronics kill) Current brightness (W/sr).....Several x Several x 10 17b (potential (considering for about (considering unintegrated if unintegrated unintegrated components) components considered) components) Minimum penetration altitude (km) About 30 About ath~ u~ber~ in this table are ~btai ~d from the American physical Society, science and Technology of Difecfed-EnergY Weapons. f?8poff Of the American ~hyskd Society Study Group, April 1987, and apply to an advanced BMD system against a responsive threat. The estimates are neither confkmed nor denied by SDIO. SDIO has identified other BMD missions for which lower entry-level systems with lower specifications (on the order of Wlsr) would be adequate. bass um i ng perfect beam quality for a system with the characteristics of the MIRACL laser. SOURCE: Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organization, 1987 and 1988.

138 131 ear flow) with the supersonic gas flowing outward from a central 1.1-m diameter cylinder formed by stacking rings of carefully machined nozzles. The laser beam will take the form of an annulus passing just outside the radially directed nozzles. A complex aspheric mirror system will keep the laser beam within this narrow ring. The goal of this program is to demonstrate multi-megawatt, near-diffraction limited operation in The brightness of a 4-m diameter (the size of the Large Aperture Mirror Program mirror), perfect, diffraction-limited beam 41 from, for example, a 1-MW laser, would be over watts/ steradian (W/sr). The Alpha laser was designed to be scaled to significantly higher levels by stacking additional amplifier segments. It would take a coherent combination of many such lasers to make a weapon able to engage a fully responsive missile threat. Chemical lasers to meet a responsive Soviet missile threat would need brightnesses of W/sr. The level needed would depend on the target dwell and retarget times. These times, in turn, depend on the laser constellation size and geometry, booster burn time and hardness, and number of targets which must be illuminated per unit time. If the Soviets were to increase the number of ICBMs in a particular launch area or decrease booster burn times, then the laser brightness needed would increase. The brightness of aground-based FEL would have to increase by a factor of 4 to 10 to account for energy losses as the beam passed through the atmosphere and travelled to and from relay mirrors in space. Several free electron lasers have been built. None has operated within a factor of 100 million (10 8 ) of the lethal brightness levels needed for a fully-responsive BMD system. Part of the reason is the low repetition rate of the pulses in experimental machines. For example, one experiment ran with the accelerator operated at a rate of one electron beam pulse every two seconds. Future accelerators will probably increase this rate to See American Physical Society, op. cit., footnote 1, p thousands of pulses per second. This will increase average brightnesses accordingly, although, as previously discussed, several more factors of 10 improvement would be needed. Lawrence Livermore National Laboratory is conducting experiments with an FEL based on an induction linear accelerator (linac). Boeing Aerospace is constructing an RF linac FEL, based on technology developed by Boeing and Los Alamos National Laboratory. Initial experiments on the Livermore FEL in 1985 produced microwave beams at 8.6 mm wavelength with peak powers of 100 MW. More recently, the peak power risen to 1.8 GW (1.8x10 9 W), 42 although this intensity lasts for only 15 nanoseconds (15x10-9 s); the average power at the repetition rates of one shot every 2 seconds was only 14 W. Scaling to shorter wavelengths demands higher quality and very high-energy electron beams. Livermore Laboratory achieved FEL lasing at 10 µm in the far IR with its Paladin laser experiment in late Boeing and a TRW/Stanford University collaboration have operated 0.5 µm visible lasers, but at low average power levels. The Boeing RF linac FEL has the advantage of multiple optical passes through the wiggler of the optical oscillator. This means that high gain is not necessary, as it is with single-pass induction linacs. 43 The RF linac also has more tolerance of variations in electron beam quality or emittance. The emittance of the RF linac electron beam could grow (i.e., deteriorate) by almost a factor of 10 without deleterious effects. In contrast, the induction linac electron beam cannot increase in emittance by more than a factor of two without degrading optical beam brightness. 44 However, there has been more uncertainty as to whether RF linacs could be scaled to the high current levels needed for BMD. Induction linacs, on the other hand, 4ZAn&ew M. Sessler and Douglas Vaughan, Free-Electron Lasers, Amen can Scientist, vol. 75, January-February, 1987, p $The RF Iinac fight require single-pass amplifiers in addition to their multi-pass oscillators (MOPA configuration) to achieve weapons-class power levels. 44 Private communication, John M.J. Madey, 1987.

139 132 have inherently high-current capability. Recently, the two FEL concepts have appeared on the whole to compete closely with one another. Excimer lasers have been utilized for lower power research and some commercial applications. The UV energy from an excimer laser is generally more damaging than visible or IR energy. However, UV light can also damage mirrors and other optical components within the laser system, making high-power operation much more difficult. Scaling to higher power is possible, but SDIO has judged the excimer program less likely to succeed, and has cut it back. The Air Force ASAT program is funding continued excimer laser research jointly with SDIO. Los Alamos National Laboratory researchers have conducted NPB-related experiments on their ATS. They have produced a current level of 0.1 A at 5 MeV. Rocket-borne tests of parts of a NPB system were planned for the late 1980s. The SDIO had planned a series of full space tests to begin in the early 1990s, including a NPB accelerator with a target satellite and a neutron detector satellite as part of the interactive discrimination experimental program. Recently, scheduling of these tests has been delayed due to funding constraints. Key DEW Issues. With such a wide gap between operational requirements and the current status of DEW, many key technical issues remain. DEW research over the next 10 to 20 years could resolve some issues judged crucial today, but could also uncover other, unforeseen, roadblocks. Some of the current issues of concern (large mirrors, pointing and tracking, and lethality measurements) are generic to all laser systems, while others are specific to particular weapon systems. Large Mirrors. All laser systems (except the x-ray laser) need very large mirrors to focus the beam to a small spot at the target. 45 b Spot size k inversely proportional to mirror diameter. Laser brightness, the primary indicator of weapon lethality, increases as the square of mirror diameter. Thus doubling the mirror size from 2 meters to 4 meters would increass laser brightness by a factor of 4. This is true for both ground-based lasers with multiple relay mirrors in space and for spacebased lasers with the mirror adjacent to the laser. In either case, the size of the last mirror (closest to the target) and its distance from the target determine the size of the laser spot focused on that target. To achieve the brightness levels of to W/sr for BMD against a fully responsive threat, laser mirrors would have to be at least 4 m (assuming mirrors were ganged into coherent arrays), and preferably 10 to 20 m, in diameter. The largest monolithic telescope mirrors today are about 5 m in diameter (Mt. Palomar), and the largest mirror built for space application is the Hubble Space Telescope at 2.4 m. The Hubble or Palomar mirror technologies would not simply be scaled up for SD I applications. The current trend both in astronomy and in military applications is to divide large mirrors into smaller segments. Electro-mechanical actuators within the mirror segments adjust their optical surfaces so that they behave as a single large mirror. Even for these segments, direct scaling of old mirror manufacturing techniques using large blocks of glass for the substrate is not appropriate: these mirrors must weigh very little. They must be polished to their prescribed surface figure within a small fraction of the wavelengths they are designed to reflect. Brightness and precision make opposite demands: usually, a thick and relatively heavy substrate is necessary to keep good surface figure. SDIO has developed new technologies to reduce substrate weight substantially. Two segments of a 3-segment, 3-m mirror (HALO) have been built. The 7-segment, 4-m mirror (LAMP) is now assembled and currently being tested. One segment of a l0-m mirror is to be built by 1991, but there are no current plans to assemble a complete 10-m mirror. Recently, the SDIO has begun tests of the lightweight LAMP mirror, designed for spacebased lasers. Durable, high-reflectivity mirror coatings are essential to prevent high laser power from damaging the mirrors. The largest mirror that

140 133 has been coated with a multi-layer dielectric coating to withstand high energy-density levels is 1.8 m in diameter. Multi-layer dielectric coatings are generally optimized to produce maximum reflectivity at the operating wavelength. Their reflectivity at other wavelengths is low (and transmission is high), meaning that off-wavelength radiation from another (enemy) laser could penetrate and damage them. 46 These coatings may also be susceptible to high-energy particle damage in space, either natural or man-made. Finally, the optical industry must develop manufacturing techniques, infrastructure, and equipment to supply the hundreds of large mirrors for BMD DEW deployment. The SDI research program has targeted mirror fabrication as a key issue, and progress has been good in the last few years. Techniques have been developed to fabricate light-weight, segmented mirrors with hollow-cored substrates and actuators to move each segment to correct for surface figure errors. These active mirrors could correct both for large-scale manufacturing errors and for operational changes such as distortions due to thermal warping. They could even correct for broad phase errors in the laser beam. The price would be added complexity. A complex electro-mechanical-optical servo system would replace a simpler static mirror. And, to make the necessary corrections, another complex wave-front detection system would measure the phase distortions of the laser beam in real time. With reliable active mirrors, it might be possible to coherently combine the output energy from two or more lasers. The brightness of N lasers could theoretically be increased to N 2 times that of a single laser with this coherent addition. (See section below on chemical lasers for more details.) a Dielectr i c coatings are nominally transmissive off the main wavelength band, but there are always defects and absorbing centers that absorb energy passing through, often causing damage and blow-off. At best the transmitted energy would be deposited in the substrate, which would then have to be designed to handle the high power density of offensive lasers. Pointing, Tracking and Retargeting Issues. A DE W beam must rapidly switch from one target to the next during a battle. Assuming that each DEW battle-station within range of Soviet ICBMs would have to engage 2 ICBMs per second, 47 then the beam would have to slew between targets in 0.3 s to allow 0.2 s of actual laser dwell time. In addition, the mirror would have to move constantly to keep the beam on the target: the target would move 1.4 km during 0.2s exposure, and the beam would have to stay within a 20- to 30-cm diameter spot on the moving target. Large 10- to 30-meter mirrors could move continuously to track the general motion of a threat cloud, but jumping several degrees to aim at a new target in 0.3 s would be rather difficult. 48 One solution would be to steer a smaller, lighter-weight secondary mirror in the optics train, leaving the big primary mirror stationary. This approach would yield only limited motion, since the beam would eventually walk off the primary mirror; in addition, the smaller secondary mirror would be exposed to a higher laser intensity, making thermal damage more difficult to avoid. Alternatively, small-angle adjustments could be made with the individual mirror segments that would constitute the primary mirrors. These mirror segments would probably have mechanical actuators to correct for gross beam distortions and thermal gradient-induced mirror warpage. Again, moving individual mirror segments would produce only limited angular motion of the total beam. 49 4Tof a laser battle-station fleet of 120, perhaps 10 to 12 would be within range of the missile fields. Assuming that average Soviet booster bum times were in the range of 130 seconds by the time a DEW system could be deployed, and allowing 30 seconds for cloud break and initial track determination, then each DEW platform in the battle space would have to engage an average of about 130 ICBMS in 100 seconds. This required targeting rate of about 1.3 per second could be increased by factors of 2 to 5 or more if the Soviet Union decided to deploy more ICBMS, and if they concentrated those extra ICBMS at one or a few sites. For this discussion, a figure of 2 per second is taken. 4aSlewing requirements cm be minimized by using appropriate algorithms. 4BAl~rnate Concepts are being kvestigated to wow retmgeting at large angles with steerable secondary mirrors.

141 134 The mirror servo system would have to accomplish these rapid steering motions without introducing excessive vibration or jitter to the beam. To appreciate the magnitude of the steering problem, consider that a vibration that displaced one edge of a 10-m mirror by 1 micrometer (1 µm 40 millionths of an inch or twice the wavelength of visible light) would cause the laser to move one full spot diameter on the target. 50 This small vibration would cut the effective laser brightness in half. Allowable jitter is therefore in the 20 nanoradian, or one part in 50 million, range. Since any servo system would undoubtedly exceed these jitter limits immediately after switching to a new target, there would be a resettling time before effective target heating could begin. This resettling time would further decrease the allowable beam steering time, say from 0.3 s to 0.2 or 0.1 s. Non-inertial methods of steering laser beams are under investigation. For example, a beam of light passing through a liquid bath which contains a periodic acoustical wave is diffracted at an angle determined by the acoustical frequency. By electronically changing the acoustical frequency in the fluid, the laser beam could be scanned in one direction without any moving parts. Two such acousto-optic modulators in series could produce a full twodimensional scanning capability. Alternatively, the laser beam could be reflected off an optical grating that diffracted it at an angle that depended on its wavelength. If the laser wavelength could be changed with time, then the beam could be scanned in one direction. Most of these non-inertial scanning techniques could not operate at weapons level laser power without damage. Others place constraints on the laser, such as limiting tunability. Approximate beam steering and retargeting levels are summarized in table 5-5. These parameters would vary with specific weapons design, system architecture, and assumed threats. In general, demands on beam steer- Whis assumes a 20-cm spot diameter on a target 1,000 km away, or an angular motion of 200 nanoradians. Table 5-5. Possible Beam Steering and Retargeting Requirements for Boost-Phase Engagement Retargeting rate targets/second Retarget time to 0.2 seconds Jitter resettling time to 0.2 seconds Average laser dwell time seconds Laser angular beamwidth a nanoradians Allowable beam jitter nanoradians ath e diffraction. [imited beam spread for a 1 ~m laser with a 10 m diameter mirror is 120 nr, SOURCE: Office of Technology Assessment, ing speed and precision would increase if the DEW range were extended (by deploying fewer than the 120 battle stations assumed here, for example), or if the Soviets increased the offensive threat above 1,400 ICBMs with average burn time of 130 s assumed above. Beam steering and retargeting needs for post-boost and midcourse battle phases could be more stressing if boost-phase leakage were high and discrimination were not reasonably effective. In general there would be more time for midcourse kills, and more DEW platforms would engage targets, but the hard-shelled RVs would withstand much more laser irradiation and hence impose longer dwell times. Lasers do not appear likely candidates for midcourse interception of RVs. A neutral particle beam weapon (NPB) would not have to dwell longer on RVs than on boosters or PBVs, since energetic particles would penetrate the RV. Without midcourse discrimination, the NPB system might have to kill from 50,000 to 1,000,000 objects surviving the boost phase, and a weapon platform would have to kill an average of 3 to 50 targets per second. At the other extreme, with effective discrimination, each NPB platform in the battle might have to engage only one RV or heavy decoy every 20 s. 51 Atmospheric Turbulence and Compensation for Thermal Blooming. -One current DEW candidate is the ground-based free electron la- 61 A55me 6,()()() RVS, 6,()()0 heavy decoys, and 10 Percent 1e*- age from the boost phase defense. If the discrimination system reliably eliminated all light decoys and debris, then, with 30 of the 120 DEW platforms in the midcourse battle, each platform would engage, on the average, one target every 22.5 seconds.

142 135 ser. The beam from this laser would be directed to a mirror in space that would reflect the beam to fighting mirrors closer to the targets. The laser beam would be distorted in passing through the atmosphere, for the same reason that stars twinkle. If not corrected, atmospheric distortion would scramble the beam, making it impossible to focus with sufficient intensity to destroy ICBM boosters. Techniques have been developed to measure this distortion of the optical wave front and to modify the phase of low power laser beams to nearly cancel the effects of the turbulent atmosphere. To correct distortion, the mirror is manufactured with a flexible outer skin or with separate mirror segments. Mechanical actuators behind the mirror surface move it to produce phase distortions that complement phase errors introduced by the atmosphere. This rubber mirror must continuously adjust to cancel the effects of atmospheric turbulence, which varies with time at frequencies up to at least 140 hertz (cycles per second). To measure atmospheric distortion, a test beam of light must be transmitted through the same patch of atmosphere as the high power laser beam. For the BMD application, this test beam would be projected from a point near the relay mirror in space, or a reflector near that relay mirror would return a test beam from the ground to the wave-front sensing system. Signals derived from the wave-front sensor computer in response to the test beam would drive the mirror actuators to correct the high-power laser beam. The wave-front sensor must generate a coherent reference beam to compare with the distorted beam, as in an interferometer. One technique, called shearing interferometry, causes two slightly displaced versions of the incoming distorted image to interfere. A computer then deduces the character of the distorted wave front by interpreting the resulting interference fringes, Another wave-ront sensor system under investigation filters part of the incoming reference beam to produce a smooth, undistorted wave front. This clean wave front can then be combined with the distorted wave front, producing interference fringes that more clearly represent the atmospheric distortion. Unfortunately the energy levels in the filtered wave front are too low, so an operational system might need image intensifiers. Atmospheric compensation of low power beams has been demonstrated in the 1aboratory and in tests during late 1985 at the Air Force Maui Optical Station (AMOS) in Hawaii. In this test, an argon laser beam was transmitted through the atmosphere to a sounding rocket in flight. A reflector on the sounding rocket returned the test signal. Wave-front errors generated on Maui drove a rubber mirror to compensate for the turbulence experienced by a second Argon laser beam aimed at the rocket. A set of detectors spaced along the sounding rocket showed that this laser beam was corrected to within a factor of two of the diffraction limit. Successful atmospheric compensation will entail resolution of two key issues: thermal blooming and fabrication of large, multielement mirrors. As a high-power laser beam heats the air in its path, it will create additional turbulence, or thermal blooming, which will distort the beam. At some level, this type of distributed distortion cannot be corrected. For example, if thermal blooming causes the laser beam to diverge at a large distance from the last mirror, then the test beam returning from the relay satellite would also spread over a large area and would not all be collected by the wave-front sensor. Under these conditions, complete compensation would not be possible. Laboratory tests of thermal blooming were planned at MIT s Lincoln Labs and fieldtesting was planned for early 1989 using the high-power MIRACL laser at the White Sands Missile Range. The latter series of tests is on hold due to lack of funding. The mirror for a BMD FEL would need 1,000 to 10,000 actuators for effective atmospheric compensation. 52 Experiments to date have used cooled mirrors with a relatively small 52 American Physical Society, op. cit., footnote 1, p. 190.

143 136 number of elements, and Itek is currently building a large uncooled mirror with many more. Nonlinear optical techniques may offer an alternative to the active-mirror correction of atmospheric turbulence. Laboratory experiments at low power have already demonstrated beam cleanup by stimulated Brillouin scattering, for example. In this technique, a beam of light with a wave front distorted by the atmosphere enters a gas cell. The beam passes partially through this gas and is reflected back with complementary phase distortion. This complementary or conjugate phase exactly cancels the phase distortions introduced by the atmosphere. The key is to amplify the phase conjugate beam without introducing additional phase errors. If perfected, this approach would eliminate moving mirror elements. Target Lethality. One term in the DEW effectiveness equation is the susceptibility of current and future targets to laser and neutral particle beams. Current U.S. missile bodies have been subjected to HF laser beams in groundbased tests, and various materials are being tested for durability under exposure to highpower laser light. 53 Laser damage varies with spot size, wavelength, pulse length, polarization, angle of incidence, and a large range of target surface parameters, making lethality test programs complex. 54 FEL beams with a series of very short but intense pulses may produce an entirely different effect than continuous HF chemical laser beams. Measuring the lethality of low-power neutral particle beam weapons intended to disrupt electronics could be more complicated. Damage thresholds would depend on the electronics package construction. However, current plans call for particle beam energy density which would destroy virtually any electronic sys- 691~ ~~~ highly publicized test at the White S~ds Missile Range, a strapped-down Titan missile casing, pressurized with nitrogen to 60 pounds per square inch pressure to simulate flight conditions, blew apart after exposure to the megawatt-class MIRACL laser. Computer models have been developed to help predict target lethality, and these models will be refined and correlated with ongoing lethality measurements. tem. 55 The kill assessment issue for NPB weapons would then become one of hit assessment: the system would have to verify that the particle beam hit the target. FEL.-The two types of FEL systems (induction linac and RF linac) face different sets of key issues (table 5-6). The induction linac FEL has the potential of very high power, but all of the laser gain must occur on one pass through the amplifier as currently designed. (Almost all other lasers achieve their amplification bypassing the beam back and forth between two mirrors, adding up incremental energy on each pass.) To achieve BMD-relevant power levels on one pass, the FEL beam diameter must be very small, on the order of a millimeter (mm). Furthermore, the beam must be amplified over a very long path, on the order of 100 m. But a millimeter-diameter beam would naturally expand by diffraction over this long path length, 56 so the induction linac must utilize the electron beam to guide and constrain the light beam while it is in the wiggler magnet amplifier, much like a fiber optic cable. This optical guiding by an electron beam has been demon- What is, the NPB would be designed to deliver 50 J/gin at the target, whereas 10 J/gin destroys most electronics (see American Physical Society, op. cit., footnoti 1, p This would assure electronics kill unless massive shielding were placed around key components. 66A l.m km of unconstrained l-pm light would expand to 120 mm after traveling 100 m. Table 5.6. Key Issues for Free Electron Lasers (FEL) For induction linear accelerator driven FELs: Electron beam guiding of the optical beam Generation of stable, high current, Iow-emittance e-beams Scaling to short wavelengths near 1 µm Raman scattering losses in the atmosphere For radio frequency accelerator-driven FELs: Scaling to 100 MW power levels Efficiency Mirror damage due to high intercavity power Cavity alignment For any FEL: Long cavity or wiggler path lengths Sideband instabilities (harmonic generation) Synchrotron/betatron instabilities (lower efficiency) SOURCE: Office of Technology Assessment, 1988,

144 137 strated, but not under weapon-like FEL conditions. Two other disadvantages derive from this narrow, intense beam of light produced by the induction linac FEL. First, the beam is so intense that it would damage any realizable mirror surface. The current plan is to allow the beam to expand by diffraction after leaving the FEL, traveling up to several km in an evacuated tunnel before striking the director mirror which would send the beam to the relay mirror in space. A second disadvantage of such intense pulses of light is that they would react with the nitrogen in the atmosphere by a process called stimulated Raman scattering. Above a threshold power density, the light would be converted to a different frequency which spreads out of the beam, missing the intended target. 57 Again, this effect could be ameliorated by enclosing the beam in an evacuated tube, allowing it to expand until the power density were low enough for transmission through the atmosphere to the space relay. On the return path to the target, however, the beam would have to be focused down to damage the target. The experimental induction linac at Livermore currently uses a (conventional, non-fel) laser-initiated channel to guide the electron beam before it is accelerated. This beam has drifted several millimeters laterally during the FEL pulse in initial experiments, severely limiting FEL lasing performance because the electron beam does not remain collinear with the FEL laser beam. The RF linac FEL, as currently configured, has shorter pulse lengths (20 picosecond v. 15 nanoseconds for the induction linac FEL) but much higher pulse repetition rates (125 MHz v. 0.5 MHz 58 ), giving it higher duty cy- S~The R~~ t~eshold for stimulated gain in nitrogen gas at one P light is about 1.8 MW/cm2. Above this power density, the atmosphere becomes a singk+pass nitrogen laser: much of the beam energy is converted to different (Stokes and anti- Stokes) wavelengths which diverge and cannot be focused on the target. athe induction linac at Livermore could be operated up to 1 khz for up to 10 pulses. An o~rational linac FEL would have a repetition rate as high as tens of kilohertz. cle and lower peak intensity for a given output average power level. It is uncertain whether the RF linac can be scaled up to produce power levels which seem probable for the induction linac. By adding a set of power amplifiers in series, it might be possible to reach the power needed for a lethal laser weapon with an RF linac FEL. The RF linac generates very high power levels inside the optical cavity. Mirror damage is therefore an issue, as is the problem of extracting energy out of the cavity at these high power levels. Cavity alignment is also critical: the mirrors must be automatically aligned to maintain path-lengths within micrometers over many tens of meters during high-power operation. The RF linac currently has low efficiency. In 1986, Los Alamos National Laboratory and a TRW-Stanford team demonstrated an energy recovery technique whereby much of the unused energy in an electron beam was recovered after the beam passed through a wiggler-amplifier. In principle, this energy could be coupled back to the RF generator to improve efficiency in an operational system. At the higher optical energy levels envisaged for the amplifiers, the RF linac amplifier should achieve 20 percent to 25 percent conversion efficiency, making energy recovery less advantageous. An FEL would tend to be fragile. Accelerators are notorious for demanding careful alignment and control, taking hours of manual alignment before operation. Major engineering developments in automatic sensing and control would be necessary before an FEL could become an operational weapon. Los Alamos is working to automate its ATS particle beam accelerator; FEL systems would have to incorporate similar automation, with the added complexity of optical, as well as accelerator, alignment. An FEL may suffer from electron beam (ebeam) instabilities. For example, unwanted longitudinal e-beam excursions could create sideband instabilities, in which part of the optical energy would be diverted to sideband frequencies. Laser light at these extraneous frequen-

145 138 cies could damage optical components designed to handle high power only at the main lasing frequency. Such sideband frequencies have been observed in FEL experiments. Lateral motion of the e-beam, called synchrotron/betatron instabilities could reduce FEL efficiency, although calculations indicate that this should not be a problem. Chemical Laser Issues. The chemical HF laser has some disadvantages relative to the FEL. Its longer wavelength (2.8-pm range) would demand larger mirrors to focus the beam on target. In general, targets would reflect a higher percentage of IR light than visible or, particularly, UV light. Hence, for a given mirror size, an HF laser would have to generate 7 to 10 times more power than an FEL laser operating at one µm, or 80 to 200 times more power than a UV laser, to produce the same power density at the target. Chemical laser experts do not believe that an individual HF laser could be built at reasonable cost to reach the to W/sr brightness levels needed for BMD against a responsive threat, since the optical gain volume is limited in one dimension by gas flow kinetics, and by optical homogeneity in the other directions. However, by combining the outputs from many HF lasers, it might be possible to produce BMD-capable HF arrays (table 5-7). These beams must be added coherently: the output from each laser must have the same fre- Table 5-7. Key Issues for the HF Chemical Laser Coherent beam combination: (many HF laser beams would have to be combined to achieve necessary power levels) Required beam brightness against a responsive threat... Reasonable HF Laser brightness for a single large unit (10 MW power and 10-m mirror) several x W/sr a 8.6 x lo Coherent Array of seven 10 MW/10-m HF lasers x10 21 ~he American Physical Society, Science and Technology of Directed-Energy Weapons: Repori of the Arnedcan Physical Society Study Group, April 1967, p. 55, estimated hardnees for a reaporwive threat to be well in excese of 10 kj/cm2. Given a range of 2,000 km and a dwell time of 0.2s, the denoted brightness is appropriate. SOURCE: Office of Technology Asae~wnent, quency and the same phase. 59 Controlling the phase of a laser beam is conceptually easy, but difficult in practice particularly at high power and over very large apertures. Since an uncontrolled HF laser generates several different frequencies in the 2.6 to 2.9 µm band, the laser array would have to operate on one spectral line, or one consistent group of lines. Three coherent coupling techniques have been demonstrated in the laboratory: 1. Coupled Resonators the optical cavities of several lasers are optically coupled, so they all oscillate in phase; 2 Injection Locked Oscillators-one low power oscillator output light beam is injected into the optical cavity of each laser; 3 Master Oscillator/Power Amplifier (MOPA) each laser is a singh-pass power amplifier fed by the same master oscillator in parallel. In one experiment, 6 CO, lasers were joined in the coupled resonator mode. With incoherent addition, the output would have been 6 times brighter than that of a single laser; with perfect coupling, the output would have been 36 times brighter. The experiment actually produced 23.4 times greater brightness. Experiments are under way to couple two l-kw, HF/DF lasers (with the coupled resonator approach) and to demonstrate MOPA operation of two HF laser amplifiers. 60 Neutral Particle Beam. Although accelerator technology is well established for groundbased physics experiments, much research, development, and testing are prerequisite to a judgment of the efficacy of a space-based particle beam weapon system. Key issues are presented in table 5-8. ~If ~d~ ~heren~y, & &am brightness of N ISSSrS wollld be Nz times the brightness of one laser. If the N lasers were not coherent, then the brightness of the combination would be the sum or N times the brightness of one laser. Actually, the MOPA experiment will utilize one amplifier with three separate optical cavities: one for the master oscillator and two for the amplifiers. (Source: SDI Laser Technology Office, Air Force Weapons Laboratory, unclassified briefing to OTA on Oct. 7, 1986.)

146 139 Photo Credit: U.S. Department of Defense, Strategic Defense Initiative Organization Artist s conception of a phased array of lasers. Since it may be impractical to build a single module spacebased chemical laser of a size useful for ballistic missile defense, scientists and engineers are exploring the possibility of using several smaller laser modules that would be phase-locked to provide a single coherent beam. This technique could increase the attainable power density on a target by a factor of N* (instead of N for incoherent addition), where N is the number of modules. Table 5-8. Neutral Particle Beam Issues Major issues: Beam divergence: 50 times improvement required Weight reduction (50 to 100 tonnes projected) Kill assessment (or hit assessment). Other issues: Beam sensing and pointing Duty factor: 100 times improvement required Ion beam neutralization (50 /0 efficient) Space charge accumulation ASAT potential SOURCE Office of Technology Assessment, 1988 The NPB ATS now at Los Alamos generates the necessary current (100 ma) for a NPB weapon, but at 20 to 40 times lower voltage, about 100 times lower duty cycle, and with about 50 times more beam divergence than would be needed for a space-based weapon. A continuous ion source with the necessary current levels has been operated at the Culham Laboratory in the United Kingdom with 30-s pulses, but not as yet coupled to an accelerator. Researchers have planned a series of groundbased and space-based experiments to develop beams meeting NPB weapons specifications. It is possible that these experiments would encounter unknown phenomena such as beam instabilities or unexpected sources of increased beam divergence, but there are no known physics limitations that would preclude weapons applications. High energy density at the accelerator would not be sufficient for a weapon. The beam would have to be parallel (or well-collimated, or have low emittance in accelerator parlance), to minimize beam spreading and maximize energy transmitted to the target. In general, higher energy beams have lower emittance, but some of the techniques used to increase beam current might increase emittance, possibly to the point where increased current would decrease energy coupled to the distant target. With high emittance, the NPB would be a short-range weapon, and more NPB weapons would be necessary to cover the battle space. The divergence of existing, centimeter-diameter particle beams is on the order of tens of microradians; this divergence would have to be reduced by expanding the particle beam diameter up to the meter range. 6l This large beam would have to be steered toward the target with meter-size magnets. Full-scale magnetic optics have not been built or tested. However, one-third scale optics have been built by Los Alamos National Laboratory and successfully tested at Argonne National Laboratory on a 50 MeV beam line. The weight of the NPB system would have to be reduced substantially for space-based operation. The RF power supply alone for a 81 In theory, beam divergence decreases as the beam size is increased. In practice, the magnets needed to increase the beam diameter might add irregularities in transverse ion motion, which could contribute to increased beam divergence; not all of the theoretical gain in beam divergence would be achieved.

147 140 weapon-class NPB would weigh 160,000 kg (160 tonnes) if based on existing RF radar technology. 62 Using solid-state transistors and reducing the weight of other components might reduce RF weight about 22 tonnes. 63 One study concluded that a total NPB platform weight of 100 tonnes is probably achievable. 64 Los Alamos scientists have estimated that the NPB platform weight for an entry level, 100- MeV, NPB system could be 50 tonnes. Some day, if high-temperature, high-current superconductors became available, NPB weights might be reduced substantially. Thermal management on a NPB satellite would be challenging. A NPB weapon might produce 40 MW of waste heat. 65 One proposal is to use liquid hydrogen to dispose of this heat. About 44 tonnes of hydrogen could cool the NPB for 500 s. 66 The expulsion of hydrogen gas would have to be controlled, since even a minute quantity of gas diffused in front of the weapon could ionize the beam, which would then be diverted by the Earth s magnetic field. Since the hydrogen gas would presumably have to be exhausted out opposing sides of the spacecraft to avoid net thrust, it might be difficult to keep minute quantities of gas out of the beam. A state-of-the-art ion accelerator (the Ramped Gradient Drift Tube Linac) can raise beam energy about 4 MeV per meter of acceler The vacuum-tube (klystron) RF power supply for the PAVE PAWS radar system weighs approximately 2 g/w of power. A NPB weapon wo~d emit an average power of 20 MW (2x10 7 watts), assuming 200 MeV beams at a current of 0.1 A. Assuming an overall efficiency of 25 percent (50 percent accelerator efficiency and 50 percent beam neutralization efficiency), the power supply would have to generate 80 MW average power, and would weigh 160 tonnes. This assumes that the RF power is generatid with l-kw, commercial quality power transistors (80,000 transistors would be required for the hypothetical 80 MW supply). These transistors can only be operated at 1 percent duty factor. New cooling technology wo~d have to be developed to operati at the 100 percent duty factor required for a NPB weapon. (See American Physical Society, op. cit., footnote 1, pp. 149 and 361.) The overall efficiency of these power supplies would be 40 percent. Ibid., p Assuming a zoo.d, loo.itiev beam, 50 percent neutr~ization efficiency, and 50 percent power generation efficiency. 66Assuming heat of vaporization OIlly (450 J/g)* ~d no temperature rise in the hydrogen. If the gas temperature were allowed to rise by 100 K, then the hydrogen mass could be reduced to about 14 tonnes. ator length. At this gradient rate, a 200-MeV beam would have to be over 50 m long. This accelerator could be folded, but extra bending magnets would increase weight and could reduce beam quality. The gradient could be increased, but if the ion beam energy were increased in a shorter length, then there would be more heating in the accelerator walls. This implies another system trade-off: reducing length in an attempt to cut weight might eventually reduce efficiency, which would dictate heavier RF power elements and more coolant. Again, future superconductors might ameliorate this problem. The beam would have to be steered to intercept the target. A NPB would have two advantages over laser beams: the convenience of electronic steering and a lesser need for steering accuracy. Magnetic coils could steer negatively charged hydrogen ions before the extra electrons were stripped off. However, the angular motion of electronic steering would be limited: the entire accelerator would have to maneuver mechanically to aim the beam in the general direction of the target cluster. Like laser weapons, a NPB must have an agile optical sensor system to track targets. However, the divergence of the NPB is larger than most laser beams (microradians versus 20 to 50 nanoradians), so the beam steering need not be as precise. On the other hand, a hydrogen beam could not be observed directly. The particle beam direction is detected in the laboratory by placing two wires in the beam. The first wire casts a shadow on the second wire placed downstream. By measuring the current induced in this downstream wire as the upstream wire is moved, the beam direction can be estimated to something like 6 microradian accuracy. New techniques would be needed to sense the beam direction automatically with sufficient accuracy. One approach utilizes the fact that about 7 percent of the hydrogen atoms passing through a beam neutralization foil emerge in a metastable excited state: the electrons of these atoms acquire and maintain extra energy. Passing a laser through the beam can make these excited atoms emit light. The

148 141 magnitude of this fluorescence depends on the angle between the particle beam and the laser beam. Thus the NPB direction can be deduced and the beam boresighted to an appropriate optical tracking system. Laboratory tests have demonstrated 250 microradian accuracy, compared to the l-microradian accuracy necessary. 67 More recent tests at Argonne at 50 MeV have yielded better results. The current technique to neutralize the hydrogen ions is to pass them through a thin foil or a gas cell. This process strips off, at most, 50 percent of the electrons, cutting the efficiency of the system in half and thus increasing its weight. A gas cell is not practical for space applications. A stripping foil must be extraordinarily thin (about.03 to.1 µm, or ten times less than the wavelength of visible light). In the proposed NPB weapon, a thin foil 1 m in diameter would have to cover the output beam. Clearly such a foil could not be selfsupporting, but Los Alamos scientists have tested foils up to 25 cm in diameter that are supported on a fine wire grid. This grid obscures about 10 percent of the beam, but has survived initial tests in beams with average power close to operational levels. Another beam neutralization concept is to use a powerful laser to remove the electrons-a technique that some assert may yield 90 percent efficiency. However, the laser stripping process would call for a 25 MW Nd:YAG laser (near weapon-level power itself), and it would eliminate the excited state hydrogen atoms needed for the laser beam sensing technique. 68 Charged hydrogen ions that escaped neutralization might play havoc with an NPB satellite. The accumulation of charge might severely degrade weapon system performance in unforeseen ways, although NPB scientists are confident that this would not bean issue. 69 The Beam Experiment Aboard Rocket (BEAR) experiment with an ion source and the planned 67 See American Physical Society, op. cit., footnote 1, p, See American Physical Society, op. cit., footnote 1, p agone ~ugge9ted that the neutralizing foil be thicker so that two electrons are stripped from some hydrogen ions, forming positive hydrogen ions (protons) to help neutralize the charge in the vicinity of the spacecraft. Integrated Space Experiment (ISE) should answer any remaaining doubts about space-charge accumulation. Arcing or electrical breakdown that could short out highly charged components may also be a problem in space. Dust or metal particles generated in ground-based accelerators fall harmlessly to the ground. In space, floating particles could cause arcing by forming a conducting path between charged components. Existing accelerators demand many hours of careful manual alignment before an experiment. Neutral particle beam weapons would have to operate automatically in space. Current plans call for the ATS accelerator at Los Alamos to be automated soon. Kill assessment might be difficult for weak particle beam weapons. Damage deep inside the target might completely negate its function with no visible sign. The choices would be either to forgo kill confirmation or to increase NPB energy levels until observable damage were caused, possibly the triggering of the high-energy explosive on the RV. The current plan is to forgo kill confirmation per se, but to increase the NPB power level to assure electronic destruction. Sensors would determine that the particle beam had hit each target. Experiments are planned to assess whether UV light emissions would indicate that a particle beam had struck the surface of a target. The planned (and now indefinitely postponed) ISE illustrates a point made in chapter 11 of this report: many BMD weapons would have ASAT capabilities long before they could destroy ballistic missiles or RVs. The ISE accelerator, if successful, would have ASAT lethality at close range, although for a limited duty cycle. Beam divergence might limit range, but it could probably destroy the electronics in existing satellites within 500 to 1,000 km. 70 Even though not aiming a beam This experiment could have nearly BMD-level lethality, possibly raising issues with respect to the ABM Treaty. However, it would not have the necessary beam sensing and pointing or the computer software and hardware for a BMD weapon; SDIO considers the experiment to be treaty compliant.

149 142 at other than a target satellite, this experiment conceivably might disrupt nearby satellite electronics. Although this may not be serious (calculations indicate that it should not), there is enough uncertainty to cause ISE planners to ask whether they should wait until the Space Shuttle had landed before turning on the ISE. X-ray Laser. The nuclear bomb-driven x- ray laser is the least mature DE W technology. To date this program has consisted of theoretical and design work at Livermore National Laboratory and several feasibility demonstration experiments at the underground Nevada nuclear weapons test site. Actual x-ray generation technology may or may not reach suitable levels in the years ahead; currently the methods to convert this technology into a viable weapons system remain paper concepts. POWER AND POWER CONDITIONING The average electrical power consumed by some proposed BMD spacecraft during battle might be factors up to 100,000 over current satellite power levels. Most existing satellites are powered by large solar arrays that would be vulnerable to defense suppression attack. To provide sufficient survivable power for space applications, most BMD satellites would require either nuclear reactors, rocket engines coupled to electrical generators, or advanced fuel cells. In addition to high average power, some proposed weapon satellites would demand high peak power: energy from the prime source, either a nuclear reactor or a rocket-driven turboalternator, would have to be stored and compressed into a train of very high current pulses. For example, a railgun might expend 500 MJ of energy in a 5-millisecond (ins) pulse, or 100 GW of peak power. This is about 1,000 times more than current pulse power supplies can deliver. The following sections outline satellite power demands and the technologies that might satisfy them. While space systems would call for the primary advances, ground-based FELs would also depend on advances in pulsed-power supply technology. Some of the technology developed for space-borne neutral particle beam systems, such as RF power sources, might be applicable to FELs. Space Power Requirements Estimates of power needs of space-based BMD systems are summarized in table 5-9. Since most of these systems have not been designed, these estimates could change significantly: the table only indicates a possible range of power levels. Power is estimated for three modes of operation: base-level for general satellite housekeeping and continuous surveillance operations lasting many years; alert-level in response to a crisis, possibly leading to war; Table 5-9. Estimated Power Requirements for Space Assets (average power in kilowatts) Mode of operation Base Alert Burst (battle) BSTS o 4-1o 4-1o SSTS (IR) Ladar Ladar imager Laser illumination o 5-1o Doppler Iadar SBI carrier o-1oo Chemical laser Fighting mirror NPB/SBFEL ,000-10, , ,000 EML (railgun) ,000-10, ,000-5,000,000 SOURCE: Space Defense Initiative Organization, 1988.

150 I143 and burst-mode for actual battle, which may last hundreds of seconds. In addition to average-power and survivability perquisites, a space-based power system would have to be designed to avoid deleterious effects of: thrust from power-generating rockets upsetting aiming, torque due to rotating components, rocket effluent disrupting optics and beam propagation, vibration on sensors and beam steering, thermal gradients, and radiation from nuclear reactors. Power systems would also have to operate reliably for long periods unattended in space. Space Power Generation Technology There are three generic sources of electrical power in space: solar energy, chemical energy, and nuclear energy. Solar Energy Solar panels have supplied power for most satellites. The sun produces about 1.3 kw of power on every square meter of solar array surface. An array of crystalline silicon cells converts the sun s energy into direct electrical current through the photovoltaic effect, with an efficiency of about 10 percent. Thus a l-m 2 panel of cells would produce about 130 watts of electricity, assuming that the panel were oriented perpendicular to the sun s rays. A 20- kw array, typical for a BMD sensor, would then have about 150 m 2 roughly, a 12-m by 12-m array. The Skylab solar array, the largest operated to date, produced about 8 kw. NASA has built, but not yet flown in space, a 25-kW experimental solar array designed to supply space station power. The major disadvantage of solar arrays is that their large size makes them vulnerable to attack. Crystalline silicon photovoltaic cells are also vulnerable to natural and man-made radiation. One approach to reduce both vulnerabilities to some degree would be to concentrate the sun s rays with a focusing optical collector. The collector would still be vulnerable, but if the system efficiency could be improved, then the area of the collector would be smaller than equivalent ordinary solar cell arrays. There are two other ways to convert the energy from solar collectors into electricity. One is to use solar thermal energy to drive a conventional thermodynamic heat engine. The other is to focus sunlight on more radiationresistant and higher-efficiency photovoltaic cells such as gallium arsenide. Depending on the temperature of the working fluid in a thermodynamic heat engine cycle, efficiencies of 20 percent to 30 percent might be achieved. Gallum arsenide cells have shown up to 24 percent efficiency in the laboratory, so 20 percent efficiency in space may be reasonable. Thus, either technology could cut the required collector area in half compared to conventional solar cells, or 75 m 2 per 20-kW output. Neither approach has been tested in space, but NASA is pursuing both for future space applications. Nuclear Energy Nuclear energy has also been used in space. There are two types of nuclear energy sources: radioactive isotope generators that convert heat from radioactive decay to electricity, and nuclear fission reactors. Both have flown in space, but the radioactive isotope generator is more common. Both radioactive decay and a controlled fission reaction produce heat as the intermediate energy form. This heat can be converted into electricity by static or dynamic means. A static power source produces electricity directly from heat without any moving parts, using either thermoelectric or thermionic converters. These converters generate direct current between two terminals as long as heat is supplied to the device. The efficiency and total practical power levels are low, but for applications of less than 500 W, the advantage of no moving parts makes a radioisotope thermoelectric generator (RTG) a primary candidate for small spacecraft. To produce more than 500 W, a radioisotope source could be coupled to a dynamic heat en-

151 144 gine. One dynamic isotope power system (DIPS) with 2 to 5 kw output has been groundtested. This system weighs 215 kg. However, the U.S. production capacity for radioactive isotopes would limit the number of satellites that could be powered by DIPS. BMD satellites needing more than 5 to 10 kw of power might carry a more powerful nuclear fission reactor. Static thermoelectric converters would still convert the heat to electricity. This is the approach proposed for the SP-100 space power program, the goal of which is to develop elements of a system to provide power over the range of 10 to 1,000 kw. The Departments of Defense and Energy and NASA are producing a reference design incorporating these elements to produce a 100-kW test reactor. 71 This is the major focus for the next generation of space power systems. The SP-100 reactor, as currently designed, would use 360 kg of highly enriched uranium nitride fuel with liquid lithium cooling operating at 1,3500 K. This heat would be conducted to 200,000 to 300,000 individual thermoelectric elements which would produce 100 kw of electricity. The overall efficiency of the system would be about 4 percent, which would entail the disposal of 2.4 MW of waste heat. Large fins heated to 8000 K would radiate this heat into space. The SP-100 program faces numerous challenges. In addition to being the hottest running reactor ever built, the SP-100 would be the first space system to: use uranium nitride fuel, be cooled by liquid lithium, use strong refractory metals to contain the primary coolant, ž have to start up with its coolant frozen, have two independent control mechanisms (for safety), and use electronic semiconductors under such intense heat and radiation stress riginal SDIO plans called for designing a 300 KW system, but as of this writing the goal has been reduced to 100 KW. *See Eliot Marshall, DOE s Way-out Reactors, Science, 231:1359, March 21, The estimated mass of the SP-100 is 3,000 kg, or a specific mass of 30 kg/kw. Original plans called for building a ground-test prototype SP-100 based on the 100-kW design by 1991, with a flight test several years later. Subsequently a 300-kW design was considered which would have pushed initial hardware toward 1993, but current schedules are fluid due to uncertain funding. To produce power levels in excess of a few hundred kw, one would have to take the next step in the evolution of space nuclear power systems: a nuclear reactor coupled through a dynamic heat engine to an electrical generator. In principle, large reactors in space could generate hundreds of MW, satisfying the most stressing BMD average power demands. A multimegawatt, or MMW, project has begun to study some of the fundamental issues raised by large reactors, including daunting engineering challenges such as high temperature waste heat disposal in space, safety in launch, operation, and decommissioning. These large nuclear systems might have to be

152 745 operated open-cycle, requiring much fuel in the form of cooling gas to dispose of excess heat. At this writing the MMW project is in the conceptual phase with no well-defined research program. Multi-megawatt nuclear reactors in space would have to be considered a 20-to-30 year project. In summary, space nuclear power systems would require extensive development to achieve reliable space operation at the kw level by the mid-to-late 1990s. Given current engineering and budget uncertainties, development of megawatt-class nuclear power systems for space cannot be projected until well into the 21st century. Chemical Energy Satellites frequently employ chemical energy in the form of batteries, fuel cells, and turbogenerators. Batteries would be too heavy for most BMD applications, except possibly for pop-up systems with very short engagement times. Fuel cells, which derive their power by combining, e.g., hydrogen and oxygen, are under active consideration for driving the accelerators of NPB weapons. For the short bursts of MMW power needed by some BMD weapons, an electrical generator driven by a rocket engine (e.g., burning liquid hydrogen and liquid oxygen) might be the only available technology in the foreseeable future. The Space Shuttle main engine (SSME) develops about 10 GW of flow power, which could generate 5 GW of electrical energy if it could be coupled to a turboalternator. Alternatively, rocket exhaust could, in principle, be converted to electricity by magnetohydrodynamics (MHD). The engineering challenges of using rocket engines to produce electrical power on board a BMD satellite are posed not only by the generator itself, but by its effects on other components such as sensors, electronics, and weapons. Two counter-rotating and counter-thrusting rockets would probably be essential to cancel torque and thrust. Even then, sensors and weapons-aiming devices would have to be isolated from vibration. Similarly, the effluent from the rocket engines must not interfere with sensors or weapon beam propagation, and electrical noise must not interfere with communication or data processing electronics. It might be necessary to place rocket engines and power generators on separate platforms hundreds or a few thousands of meters away to achieve the necessary isolation, transmitting power by cable or microwaves. This method, however, would raise vulnerability issues, presenting to the adversary an additional target and a vulnerable umbilical cord. Power Conditioning Power conditioning is matching the electrical characteristics of a power source with those required by the load. A generator might produce a continuous flow of electrical current, but a load, such as railgun firing, would require a series of very high-current pulses. Power conditioning equipment would convert the continuous flow into pulses. In some cases the projected power conditioning device requirements exceed existing capabilities by two or three orders of magnitude, even for ground-based experiments. In many areas, no space-qualified hardware exists at any power level. Pulsed power technology development efforts are underway in capacitive and inductive energy storage, closing and opening switches, transformers, RF sources, AC- DC converters, and ultra high-voltage techniques and components. Particle accelerators that drive the FEL and the NPB use RF power. Railgun requirements would present the greatest challenge: very short (millisecond) pulses of current several times a second. Many electrical components would have to be developed to produce the proper current pulses for a railgun. A homopolar generator combined with an inductor and opening switch is now the primary candidate for the generation of very short pulses. A homopolar generator is a rotating machine that stores kinetic energy in a rotating armature. At the time of railgun firing, brushes would fall unto the armature, extract-

153 146 ing much of its energy in a fraction of a second. This would result in a sudden jerk in the torque of the generator, which would disturb a spacecraft unless compensated by a balanced homopolar generator rotating in the opposite direction. The brushes would also wear out, which raises questions about the durability of a railgun with high repetition rates. Very fast switches would be essential. These switches would have to be light enough to move rapidly, but heavy enough to handle the extraordinarily high currents. Researchers at the University of Texas have investigated one advanced modification to the homopolar generator. They have replaced brushes and switches with inductive switches in a compulsator, a generator which produces a string of pulses. By replacing noncurrent carrying iron with graphite-epoxy composites, these compulsators could be much lighter than the homopolar generators. While space applications drive power development requirements, emerging ground-based defensive systems would also stress existing power sources. Ground-based BMD elements might require diesel and turbine driven electric generators and MHD generators for mobile applications. A fixed-site system such as the FEL might draw on the commercial utility grid, dedicated power plants, or superconducting magnetic energy storage (SMES). The electrical utility grid could meet peacetime housekeeping power needs and could keep a storage system charged, but, due to its extreme vulnerability to precursor attack, could not be relied on to supply power during a battle. Therefore, a site-secure MMW power system would probably be necessary. Superconducting magnetic energy storage is a prime candidate for ground-based energy storage; an SMES system would be a large, underground superconducting coil with continuous current flow. The science of SMES is well established, but engineering development remains. Recent discoveries of high-temperature superconductors could have an impact on future power supplies and pulse conditioning systems. Given the likely initial cost of manufacturing exotic superconducting materials and the probable limits on total current, their first applications will probably be in smaller devices such as electronics, computers, and sensor systems. But if: scientists could synthesize high temperature superconducting materials able to carry very large currents; and engineers could develop techniques to manufacture those materials on a large scale suitable for large magnetic coils, RF power generators, accelerator cavity walls, the rails of electromagnetic launchers, etc.; then superconductors could substantially reduce the power demand. Efficiency of the power source and power conditioning networks could also be improved. High temperature superconductors would be particularly attractive in space, where relatively cold temperatures can be maintained by radiation cooling. COMMUNICATION TECHNOLOGY Communication would be the nervous sys- dreds to several thousands of weapons plattem of any BMD system. A phase-one defense forms in low-earth orbits. would include hundreds of space-based components, separated by thousands of kilometers, Three fundamental communication paths for boost and post-boost interception. A would link these space assets: ground to space, second-phase BMD system would include space to space, and space to ground. Ground many tens of sensors in high orbits and hun- command centers would at least initiate the

154 147 battle; they would also receive updates on equipment status and sensor data in peacetime and as the battle developed. The attributes of an effective communication system would include: adequate bandwidth and range, reliability, tolerance of component damage, security from interception or take-over, tolerance of nuclear effects, and jam- or spoof-resistance. The bandwidths, or frequency space available, from links in the millimeter-wave bands would be adequate for most near-term BMD functions. The most demanding element would be the boost surveillance and tracking system (BSTS) satellite, with perhaps a l-million-bitper-second data rate. Second-phase elements such as a space surveillance and tracking system (SSTS) sensor satellite might operate at much higher rates, up to 20 million bits per second, while battle management might take 50 or more million bits per second of information flow. Various additional data for synchronization signals would have to be communicated. Transmission bandwidths might have to be very large-perhaps 1-10 gigahertz (GHz) to reduce the chances of jamming. The communication system must be durable and survivable even if some nodes fail due to natural or enemy action. Redundant links in a coupled network might assure that messages and data got through even if some satellites were destroyed. Tying together a vast BMD space network would be challenging, especially given that the satellites in low-earth orbit would constantly change relative positions. One key issue for BMD communications is jamming by a determined adversary. Successfully disrupting communications would completely negate a BMD system that relied on sensors and command and control nodes separated from weapons platforms by tens of thousands of km. Jammers could be developed, deployed, and even operated in peacetime with little risk of stimulating hostile counteraction. Space-to-ground communication links would be particularly vulnerable. Ground-based, shipbased or airborne high-power jammers might block the flow of information to satellites. In wartime, nuclear explosions could disrupt the propagation of RF waves. Ground-based receivers would also be susceptible to direct attack. Even space-based communications would be susceptible to jamming. Recently there have been two primary SDI candidates for BMD communication links in space: laser links and 60-GHz links. A 60-GHz system once seemed to promise a more jamresistant channel for space-to-space communications, since the atmosphere would absorb enough 60-GHz energy to reduce the threat of ground-based jammers. Recent analyses, however, indicate that space and air-based jammers may limit the effectiveness of 60-GHz links. 60-GHz Communication Links The operating frequencies of space communication systems have been steadily increasing. For example, the Milstar communications satellite will use the extremely high frequency (EHF) band with a 44-GHz ground-to-space uplink and a 20-GHz downlink. These high frequencies allow very wide bandwidth (1 GHz in the case of Milstar) for high data transmission rates, but also for more secure communications through wide-band-modulation and frequency-hopping anti-jamming techniques. For space-to-space links, BMD designers are considering even higher frequencies-around 60 GHz. This band includes many oxygen absorption lines. It would be very difficult for ground-based jammers to interfere with 60- GHz communications between, for example, a BSTS early warning satellite and SBI CVs: oxygen in the atmosphere would absorb the jamming energy. Pre-positioned jammer satellites, or possibly rocket-borne jammers launched with an attack, might still interfere with 60-GHz channels. The main beam of radiation from a 60-GHz transmitter is relatively narrow, mak-

155 148 ing it difficult for an adversary to blind the system from the main lobe: the enemy jammer transmitter would have to be located very close to the BMD satellite broadcasting its message. But a 60-GHz receiver would also pick up some energy from the sidelobes and even some from the opposite side of the receiver (the backlobes ). While a receiver may be 10,000 to 100,000 times less sensitive to energy from these sidelobes than from the main lobe, it must be extremely sensitive to pickup signals from a low-earth orbit satellite tens of thousands of km away. At high-earth orbit, a near-by jammer with only a few hundred watts of power could overwhelm a much more powerful 60-GHz system on a sensor satellite. This neighbor might masquerade as an ordinary communications satellite in peacetime. In wartime, it could aim its antenna at the BSTS and jam the channel. The countermeasure would be to station the BSTS out of standard communications satellite orbits. Laser Communication Links The low-power diode laser offers the possibility of extremely wideband, highly directional, and, therefore, very jam-proof communications. The MIT Lincoln Laboratory has designed a 220 megabit-per-second (Mbs) communication link that would need just 30 milliwatts of laser power from a gallium aluminum arsenide (GaAIAs) light emitting diode (LED) to reach across the diameter of the geosynchronous orbit (about 84,000 km). The receiver, using heterodyne detection, 73 could pull The common heterodyne radio receiver includes a local oscillator which generates a frequency that is combined or mixed with the incoming radio signal This process of mixing the local oscillator signal with the received signal improves the ability to detect a weak signal buried in noise, and reduces in a signal of just 10 picowatts (10-11 W) power. A 20-cm mirror on the transmitter would direct the laser beam to an intended receiver. The high directionality of narrow laser beams also complicates operation. A wideangle antenna could flood the receiver area with signal, even sending the same message to many receivers in the area at one time. A narrow laser beam must be carefully aimed at each satellite. This would require mechanical mirrors or other beam-steering optics, as well as software to keep track of all friendly satellites and to guide the optical beam to the right satellite. The lifetime of a laser source and an agile optical system may be relatively short for the first few generations of laser communication systems. A laser communication system, as presently designed, would require up to eight minutes to establish a heterodyne link between a transmitter and a receiver. Plans call for reducing this acquisition time to one minute. With a very narrow laser beam, even minute motions of the transmitter platform could cause a momentary loss of coupling, forcing a delay to reacquire the signal. While laser links might provide jam-proof communications between space-based assets of a BMD system, laser communications to the ground would have to overcome weather limitations. One approach would use multiple receivers dispersed to assure one or more clear weather sites at all times. Alternatively, one could envisage an airborne relay station, particularly in time of crisis. interference. A laser heterodyne receiver would include its own laser source, which would be mixed at the surface of a light detector with the weak light signal from a distant laser transmitter. SPACE TRANSPORTATION Reasonable extensions of current U.S. space launch capabilities would be necessary to lift transportation capability might launch the several hundred to over one thousand carrier tens of sensor satellites envisaged by some vehicles and their cargoes of thousands to tens BMD architectures, but entirely new space of thousands of kinetic kill missiles into space

156 149 in a reasonable period of time. Therefore space launch capability would have to evolve along with phase-one and phase-two weapon systems to assure the United States and to persuade the Soviet Union-that a defense-dominated world would be feasible and enduring. Space Transportation Requirements Space-based interceptors and their carrier satellites would dominate initial space deployment weights. Assuming that a phase-one deployment would include a few hundred CV\sS and a few thousand SBIs based on the "stateof-the-art rockets described above, then total launch weight requirements might be in the range of 1 million to 2 million kg. The range of weights estimated by SD I system architects for a more advanced phase varied from 7.2 to 18.6 million kg. The large range of weight estimates reflects differences in architectures, and particularly differences in survivability measures. Several contractors indicated that survivability measures such as shielding, decoys, proliferation, and fuel for maneuvering-would increase weight by a factor of about three. One could infer that the heavier designs might be more survivable. Additional space transportation would be required over time for servicing, refueling, or replacement of failed components. One unresolved issue is how best to maintain this fleet of orbiting battle stations: by originally including redundant components such as interceptor missiles on each satellite, by complete replacement of defective satellites, by on-orbit servicing, or by some combination of the above. One contractor estimated, for example, that it would take 35 interceptor missiles on each battle station to assure 20 live missiles after 10 years, with the attrition due entirely to natural component failures. Soviet countermeasures might drive up weight requirements substantially in later years. Increased Soviet ICBM deployments might be countered with more SB I platforms. Defense suppression threats such as directascent ASATs might be countered in part by proliferation of SBI battle stations or by other heavy countermeasures. Advanced decoys dispersed during the post-boost phase of missile flight might require some type of interactive discrimination system in space. Reduced Soviet booster burn times would eventually impel a shift to DE W. Deploying these countermeasures would necessitate additional space transportation capability. Directed-energy weapon components in particular would probably be very heavy. The range of SDI system architects estimates for some far-term systems was from 40 million to 80 million kg. Space Transportation Alternatives There seem to be two fundamental options for lifting the postulated BMD hardware into space: use derivatives of existing space transportation systems; or design, test, and build anew generation space transportation system. The first option might be very costly; the second might postpone substantial space-based BMD deployment into the 21st century. Some BMD advocates outside the SDIO have suggested that existing United States space launch systems might be adequate for an initial spacebased BMD deployment in the early 1990s. But the existing United States space launch capability is limited in vehicle inventory, payload capacity per launch, cost, launch rate, and launching facilities. As shown in table 5-10, today s total inventory of U.S. rockets could lift about 0.27 million kg into low-earth orbit (180 km) at the inclination angle of the launch site (28.50 for the Kennedy Space Center in Florida).74 The bulk of early SBI deployments would have to be launched into near-polar orbits from Vandenberg AFB, which would now only be possible for the 6 remaining Titan 34D vehicles with a combined lift capacity of 75,000 kg. Missile launch capacity is usually specified in terms of the payload which can be lifted into direct East-West flight at an altitude of 180 km, which produces an orbit inclined at the latitude of the launch point. Extra propellant is required to lift the payload to higher inclinations or to higher altitudes. Proposed BMD weapons systems would require higher inclinations (700 to850, and higher altitudes (600 to 1,000 km), which translates into lower payload capacity.

157 150 This would correspond to about 6 percent of the initial phase-one BMD space deployment requirements. Some have suggested refurbishing Titan-IIs, which have been retired from the ICBM fleet. If all 69 Titan-IIs were refurbished, then the United States could lift another 130,000 kg into polar orbit, or another 11 percent of the near-term BMD needs. The rate of missile launch might also be limited by the existing space transportation infrastructure. Launching one Shuttle now takes a minimum of 580 hours at the Kennedy Space Center (and might take about 800 hours at Vandenberg AFB 75 ), limiting potential launches to one per month or less from each complex. After the Shuttle accident, NASA estimated that 12 to 16 flights per year would be reasonable. Clearly 16 launches per year would not be sufficient for BMD deployment. 76 Several aerospace companies have proposed building launch vehicles with increased lift capacity to meet SDI, DoD, and civilian space transportation demands. Many of these vehicles would be derived from various Shuttle or 75Completion of the Vandenberg Shuttle launch site SLC-6 has been postponed until Assuming 16 Shuttle launches per year with 9,000 kg payload to low polar orbit, it would take between 8 to 12 years to deploy a phase-one BMD system and 48 to 125 years to deploy a phase-two system weighing 7 to 18 million kilograms. Titan predecessors, such as the Titan-4, included in table Twenty-three Titan-4s will be built by 1988, but these have only marginally increased lift capacity. A major increase in lift capacity to the 40,000 to 50,000 kg range would be required for an effective space-based BMD system. Even for a phase-one system, far more would be needed by the mid-1990s. Both SDIO and Air Force officials have called for anew space transportation system that is not a derivative of existing technology. Four aerospace companies analyzed various space transportation options under joint Air Force/NASA/SDIO direction. The Space Transportation Architecture Study (STAS) compared manned v. unmanned vehicles, horizontal v. vertical takeoff, single v. 2-stage rockets, and various combinations of reusable v. expendable components. 77 The &r Force, after reviewing the initial STAS work, appears to be leaning toward a decision that the BMD deployment should use an unmanned, expendable, 2-stage heavy-lift launch vehicle (now called the ALS or advanced launch system).78 TTThe spice Trmsportation Architecture Study (S I AS) was a joint Air Force/NASA/SDIO study on future space transportation systems. The Air Force Systems Division contracted with Rockwell and Boeing, while NASA employed General Dynamics and Martin Marietta to analyze U.S. civilian and military space requirements and possible alternatives to satisfy them. 7sThe name HLLV (heavy lift launch vehicle) was changed to ALS in April Table Current U.S. Space Launch Inventory a Payload per vehicle (thousands of kg) Inventory LEO Polar quantity (180 km) (180 km) Geo Shuttle Centaur-G:4.5 IUS: 2.3 Titan 34D , IUS: 1.8 Titan-4 b (23) Centaur-G:4.6 IUS: 2.4 Titan 11-SLV (13) C Delta Delta (MLV) (7) Atlas scout d (ALS) ? (50-70) (40-55).... aparentheses indicate future SyStemS. bth e Titan.4 or the complementary Expendable Launch Vehicle (CELV) is the latest in the line Of Titan missile configurations; 23 have been ordered. CThe Titan 11.sLvs are Ming refurbished from the ICBM inventory. The first Titan-n may be available by An additional 56 Titan II could be refurbished from the retired ICBM fleet. dth e Advanced Launch System is proposed to deploy the bulk of the BMCI SpaCe CO171pOIlWlk. SOURCE: Office of Technology Assessment, 1988.

158 151 An interim STAS study suggested that such a vehicle would have to evolve to a partially reusable system to meet SDIO cost reduction goals. The STAS contractors projected that development of a heavy-lift unmanned vehicle would require about 12 years, although at least one aerospace company estimates that an ALS could be developed in 6 years. If the original 12-year estimate is correct, significant space deployment of a BMD system could not begin until the turn of the century even if the weapon systems were ready earlier. If the 6- year estimate were correct, then initial deployment could begin by To deploy space-based assets earlier, SDIO has suggested a two-tier level program: build part of an ALS by the mid-1990s, but design this system to evolve into the long-range system by the year The initial system would include some of the advanced features of the heavy-lift launch vehicle concepts outlined by STAS, but would not have a fly-back booster and would not meet the SDIO cost goals of $300 to $600 per kilogram. The interim goal would be to reduce the current costs of $3,000- $6,000 per kilogram to $1,000-$2,000. Building a space transportation system while trying to meet these two goals simultaneously could be risky. Compromises might be required either to meet the early deployment date or to meet the long-term cost and launch rate goals. The estimated launch rate for a fully developed ALS vehicle is about once per month per launch complex. 79 Assuming a 40,000-kg payload to useful BMD orbits, then between 30 and 45 successful flights would be required for a phase-one BMD deployment and from 180 to 460 flights for a much larger second phase. Allocating 5 years to deploy the latter system, the United States would need to build three to eight new launch facilities YFhe current maximum launch rate for Titans is three per year from each pad, which might be increased to five per year. Further increases are unlikely because the Titans are assembled on-site. This is one of the reasons an entirely new space launch system would be needed to meet the SDI launch rates. * The United States now has four launch pads for Titan-class boosters, two on the east coast and two on the west coast. One west coast pad is being modified to handle the CELV. Since Photo credit: U.S. Department of Defense, Strategic Defense Initiative Organization Advanced launch system (ALS). Large-scale deployment of space-based interceptors (SBI) or other weapons in space will require a dramatic expansion of US. space-launch capabilities. Various proposals, including a Shuttle-derived, unmanned launch vehicle such as this have been under consideration by the Air Force, NASA, and the SDIO. Figure 5-13 presents one very optimistic sce nario which might lead to space launch facilities adequate for proposed second-phase BMD SBIs would have to be launched from Vandenberg to reach nearpolar orbits, all early deployments would have to be from one pad. The estimated time to build a new launch pad complex is 7 to 10 years,

159 152 Figure 5-13a. Annual Space Launch Capacity (near polar orbits at 800 km) Table 5-il. Space Transportation Vehicle capacity to 600 km, high inclination: (thousands of kilograms) CELV (Titan-4) 115 Titan 34D Earty HLLV 30 ( ) Final HLLV 44 (2000+) Year This is one possible scenario to achieve the 2 million kg per year space launch capability into near-polar orbits required for an intermediate ballistic missile defense system. This system could conceivably reach this goal by the year 2003, assuming that three new launch pads were built at Vandenberg AFB, and the proposed Heavy Lift Launch Vehicle (HLLV)/Advanced Launch System (ALS) could be developed, flight tested, and ready for initial service with 30,000 kg lift capacity by This would be 5 years ahead of the schedule initially suggested by the Space Transportation Architecture Study (STAS). The HLLV is further assumed to evolve into a 44,000 kg capability by the year 2000, without any engineering delays. The SDIO launch goals as of early 1987 are shown for comparison. SOURCE: Office of Technology Assessment, systems by This scenario assumes that the SDIO two-level space transportation development approach would be successful: an interim ALS vehicle, with a 30,000 kg capability to near-polar orbits, would be available by 1994; a more advanced ALS would come online in 2000 with 44,000 kg capacity. Three new launch pads would be built (although there is no room for three new pads at Vandenberg AFB, the only existing site in the contiguous United States with near-polar orbit capability). Assuming approval to proceed with the new launch system in 1988, the first flights of the new ALS would begin in 1994, using the refurbished SLC-6 launchpad at Vandenberg, built originally for the Space Shuttle. The three new pads would become operational in 1997,1998, and Flights would be phased in at each site, increasing up to 12 flights per year per pad. With these assumptions, the SDIO goal Total Number of launches per year annual launch Launch pads: 4-East: SLC-6 Capacity (34D/CEL) (HLLV) (HLLV) (HLLV) (HLLV) (M kg) Year Tabular data for figure 3-13a. SOURCE: Office of Technology Assessment, of 2 to 2.5 million kg per year could be achieved by If the United States were to operate 10 launch facilities, each with one ALS launch per month, then it would take about 10 years to orbit the 50 million kg estimated for a far-term, 81 third-phase system. If political or strategic considerations (such as transition stability) would not allow as long as 10 years to deploy, then the United States would have perhaps three choices: 1. develop another new vehicle with lift capacity above 50,000 kg to 800-km, high inclination orbits; 2. build and operate more than ten ALS launch facilities simultaneously; or ElThe 50 fion kg assumes the low end of the 40 to 80 ~- iion kg estimated above for phase three with spacebased lasers. A successful ground-based laser system could reduce this estimate by about 15 million kg, or 25 to 65 million kg for a total phase-three constellation.

160 improve launch operations to reduce turnaround time below 30 days per pad. The country would have to expand booster manufacturing capacity to meet this demand for up to 120 launches per year. Historically, Titan production lines completed up to 20 missiles per year, and Martin Marietta has estimated that it could easily produce 14 of the Titan class per year with existing facilities. 82 Space Transportation Cost Reduction Identifying 42 technologies related to space transportation, the STAS listed several where research might lead to reduced operating costs (it emphasized the first three as offering especially high leverage for cost reduction): lightweight materials, expert systems and automated programming to cut software costs, better organization, reducing dry weights substantially, better ground facilities, higher performance engines, fault-tolerant avionics, reusability of major components, and better mating of spacecraft to launch vehicle for reduced ground costs... szthis would inclu& 5 CELVS, 6 Titan 11s, and 3 Titan 34Ds. The operating (as opposed to life-cycle) costs of space transportation are currently estimated at $3,300 to $6,000 per kilogram of payload to low-earth orbit, and $22,000 to $60,000 per kilogram to geosynchronous orbit. At that rate, it would cost $24 billion to $200 billion to launch a phase-two BMD system, and $140 billion to $450 billion for a responsive phasethree deployment, based on the constellation weights estimated by various SDIO system architects. The SDIO has set a goal of reducing launch operating costs by a factor of 10. Operating costs are estimated at about one third of the total life-cycle costs of a space transportation system. Based on current operating costs, total life-cycle costs for transporting a phase-two BMD system into space might be $72 billion to $600 billion; for phase three, the costs might range from $420 billion to $1.35 trillion. Reaching the goal of reducing operating costs by a factor of 10 would reduce life-cycle costs for space transportation by only 30 percent. Assuming that this percentage would be valid for a new space transportation system, and assuming a 10 to 1 reduction in operating costs only, then the total life-cycle costs for space transportation might be $50 billion to $420 billion for a phase-two deployment and $290 billion to $900 billion for a phase-three deployment. Clearly the other kinds of costs for space transportation would have to be reduced along with the operating costs. CONCLUSIONS Weapon Technology Conclusions Phase One Kinetic Energy Weapons. KEWs (or else the kinds of nuclear-armed missiles developed for BMD in the 1960s) would most likely be the only BMD weapons available for deployment in this century and possibly the first decade of the 21st century. Several varieties of non-nuclear, hitto-kill KEW form the backbone of most nearand intermediate-term SDI architecture proposals. Considering the steady evolution of rockets and smart weapon homing sensors used in previous military systems, it seems likely that these KEWs could have a high probability of being able to destroy individual targets typical of the current Soviet ICBM force by the early to mid-1990s. The key unresolved issue is whether a robust, survivable, integrated system could be designed, built, tested, and deployed to intercept in the face of likely countermeasures a sizeable fraction of evolving Soviet nuclear weapons. Space-Based Interceptors. SBIs deployed in the mid to late 1990s could probably destroy

161 154 some Soviet ICBMs in their boost phase. The key issue is whether the weight of the SBI projectiles could be reduced before Soviet booster burn times could be shortened, given that existing SS-24 and SS-25 boosters would already stress projected SBI constellations. The probability of post-boost vehicle (PBV) kills is lower due to the smaller PBV size and IR signal, but SBIs might still achieve some success against current PBVs by the mid to late 1990s. Exe-atmospheric Reentry Interceptor System. The ERIS, which has evolved from previous missiles, could probably be built by the early to mid-1990s to attack objects in late midcourse. The key unknown is the method of tracking and discriminating RVs from decoys. Existing radar sensors are highly vulnerable, the SSTS space-based IR sensor probably would not be available until the late 1990s to early 2000s, and the AOS airborne sensor would have limited endurance and range. This would leave either new radars or some type of pop-up, rocket-borne IR probe, which have apparently received little development effort until recently. Given the uncertainty in sensors suitable for the ERIS system, its role would probably be confined to very late midcourse interceptions and it might have limited BMD effectiveness until the late 1990s. Phase Two High Endo-atmospheric Defense Interceptor. The HEDI could probably be brought to operational status as soon as the mid-1990s. To overcome the unique HEDI window heating problem, the HEDI on-board homing IR sensor needs more development than its ERIS cousin. But the HEDI system does not depend on longrange sensors to achieve its mission within the atmosphere. The HEDI could probably provide some local area defense of hardened targets by the mid-1990s against non-marved RVS. 83 HEDI performance against MaRVed RVs appears questionable. SBIs against Reentry Vehicles.-The probability that SBIs would kill RVs in the mid-course is low until the next century, given the difficulty in detecting and tracking many small, cool RVs in the presence of decoys, and given uncertainties in the SSTS sensor and battle management programs. Phase Three Directed-Energy Weapons. -It is unlikely that any DEW system could be highly effective before 2010 to 2015 at the earliest. No directed energy weapon is within a factor of 10,000 of the brightness necessary to destroy responsively designed Soviet nuclear weapons. (OTA has not had the opportunity to review recent SDIO suggestions for entry level DEWS of more modest capability. SDIO contends that effective space-based lasers of one to two orders of magnitude less than that needed for a responsive threat could be developed much sooner.) At least another decade of research would likely be needed to support a decision whether any DEW could form the basis for an affordable and highly effective ballistic missile defense. Further, it is likely to take at least another decade to manufacture, test, and launch the large number of satellite battle stations necessary for highly effective BMD. Thus, barring dramatic changes in weapon and space launch development and procurement practices, a highly effective DEW system is unlikely before 2010 to 2015 at the earliest. Neutral Particle Beam.-The NPB, under development initially as an interactive discriminator, is the most promising mid-course DEW. 84 Shielding RVs against penetrating particle beams, as opposed to lasers, appears prohibitive for energies above 200-MeV. Although laboratory neutral particle beams are still about 10,000 times less bright than that needed for sure electronics kills of RVs in space, the necessary scaling in power and reduction in beam divergence appears feasible, if challenging. S MaRV refers to maneuvering reentry vehicles, or RVs which can change their course after reentering the atmosphere to improve accuracy or to avoid defensive interceptors. 8iThe NPB would have virtually no boost phase capability against advanced responsive boosters since particle beams cannot penetrate below about 150 kilometers altitude.

162 155 However, as discussed in chapter 4 under the topic of NPB interactive discrimination, it is unlikely that engineering issues could be resolved before the late 1990s, which would most likely postpone deployment and effective system operation to at least Free Electron Laser. The free electron laser (FEL) is one of the more promising BMD DEW weapon candidates. The FEL is in the research phase, with several outstanding physics issues and many engineering issues to be resolved. Even if powerful lasers could be built, the high power optics to rapidly and accurately steer laser beams from one target to the next could limit system performance. Although the basic system concept for an FEL weapon is well developed, it is too early to predict BMD performance with any certainty. Chemical Laser. There are too many uncertainties to project BMD performance for the chemically pumped hydrogen fluoride (HF) laser. The HF laser has been demonstrated at relatively high power levels on the ground, although still 100,000 times less bright than that needed for BMD against a responsive threat. Scaling to weapons-level brightness would require coherent combination of large laser beams, which remains a fundamental issue. This, coupled with the relatively long wavelength (2.8 micron region), make the HF laser less attractive for advanced BMD than the FEL. Electromagnetic Launcher. There are too many uncertainties in the EML or railgun program to project any significant BMD capabilities at this time. Phase One Space Power Conclusions Power Requirements. Nuclear power would be required for most BMD spacecraft, both to provide the necessary power levels for stationkeeping, and to avoid the vulnerability of large solar panels or solar collectors. Dynamic Isotope Power System. The DIPS, which has been ground-tested in the 2 to 5 kw range, should be adequate and available by the mid to late 1990s, in time for early BSTS-type sensors. Phase Two Nuclear Reactors. Adequate space power may not be available for SSTS or weapon platforms with ladars before the year For BMD satellites that require much more than 10 kw of power the SP-100 nuclear reactor/ thermoelectric technology would have to be developed. This is a high-risk technology, with space-qualified hardware not expected before the late 1990s to early 2000s. Phase Three Chemical Power. Chemically driven energy sources (liquid oxygen and liquid hydrogen driving turbogenerators or fuel cells) could probably be available for burst powers of MW up to GW to drive weapons for hundreds of seconds by Power for Electromagnetic Launchers. -Highcurrent pulse generators for electromagnetic launchers (EML) would require extensive development and engineering, and would most likely delay any EML deployments well into the 21st century. High-Temperature Superconductors. Research on high-temperature superconductors suggests exciting possibilities in terms of reducing the space power requirements and improving power generation and conditioning efficiencies. At this stage of laboratory discovery, however, it is too early to predict whether or when practical, high current superconductors could affect BMD systems. Space Communications Conclusion Laser communications may be needed for space-to-space and ground-to-space links to overcome the vulnerability of 60-GHz links to jamming from nearby satellites. Wide-band laser communications should be feasible by the mid-1990s, but the engineering for an agile beam steering system would be challenging.

163 156 Space Transportation Conclusions Phase One Mid-1990s Deployments. Extrapolating reasonable extensions of existing space transportation facilities suggests that a limitedeffectiveness, phase-one BMD system begun in the mid-1990s could not be fully deployed in fewer than 8 years. 85 Assuming that the hardware could be built to start deployment in 1994, the system would not be fully deployed until A more ambitious launcher-development program and a high degree of success in bringing payload weights down might shorten that period. Phase Two New Space Transportation System. A fully new space transportation system would be required to lift the space assets of a phase-two BMD system. This system would have to include a vehicle with heavier lift capability (40,000 to 50,000 kg v. 5,000 kg for the Tita.n- 4), faster launch rates (12 per year v. 3 per year per pad), and more launch pads (4 v. 1). J6 l & ~Sumes that two launch pads at Vandenberg AFB, 4-East and the SLC-6 pad intended for the Shuttle, are modified to handle the new Titam4 complementary expendable launch vehicle (CELV), and the launch rates are increased from three Titans per year per pad up to six per year. Optimistic Assumptions. Even under very optimistic assumptions, 86 the new space transportation system would be unlikely to reach the necessary annual lift requirements for a largescale, second-phase BMD until , with full phase-two deployment completed in the period. Phase Three Ultimate DEW Systems. It might take 20 to 35 years of continuous launches to fully deploy far-term, phase-three BMD space assets designed to counter with very high effectiveness an advanced, responsive, Soviet missile threat. This estimate assumes deployment of the proposed ALS space transportation system and the kind of advanced space-based laser constellation suggested by SD I system architects. A set of ground-based laser installations could reduce the space launch deployment time estimate to years. ~his assumes that the SDIO bifurcated goal is met: a revolutionary space transportation system with 10 times lower cost is developed in 12 years, while a near-term component of that system yields a working vehicle of reduced capability by 1994.

164 Chapter 6 System Development, Deployment, and Support

165 CONTENTS Introduction Architecture Definition System Development System Testing Fabrication cost Time Quality Deployment Operation and Maintenance Example BMD Sub-System: SSTS Key System Issues System Testing Automatic Operation Scheduling and Deployment: An Illustrative Scenario System Conclusions Testing Automation Scheduling and Deployment Page Box 6-A. SDIO Demonstration Box Experiments Page Figure No. Figures 6-1. Number of Space-Based Interceptors Launched Into Space Number of Warheads Leaking Through Boost and Post-Boost Defenses Number of Warheads Leaking Through Boost and Post-Boost Defenses a. Number of Warheads Leaking Through Boost and Post-Boost b. Number of Warheads Leaking Through Boost and Post-Boost Defenses Table No, Tables Page 6-1. Examples of Current v. Required Manufacturing Capacity for Proposed BMD Systems SSTS Sub-Systerms Three-Color LWIR Sensor Components Assumed Distribution of Soviet ICBMs for

166 Chapter 6 System Development, Deployment, and Support INTRODUCTION The preceding chapters review the status of key ballistic missile defense (BMD) technologies, describing the progress made and the additional advances still needed to meet various BMD goals. These technologies would have to work together in an integrated system. The United States would have to develop the infrastructure to fabricate, test, deploy, operate, and maintain that system, and modify it in response to Soviet countermeasures. In the case of space-based elements, now considered essential for a highly effective defense, the United States would have to design, test, and build anew space transportation system. Anything but the fastest development of this transportation system could delay all but the most modest space-based BMD deployment to well into the 21st century. This chapter explores the steps involved in moving from the current research and development phase to operational status. These steps include: architecture definition, system development, system testing, fabrication, deployment, and operation and maintenance. Given the complexity of a global BMD system and the immaturity of many technologies, this chapter can only outline and give some indication of the multitude of challenges that would face engineers and manufacturers if a decision were made to proceed to full-scale engineering development (FSED) and then to deployment. From the beginning, the development and deployment of dependable computer software would be a key issue; the subject of software is deferred until chapter 9. ARCHITECTURE DEFINITION The first step toward deployment would be to complete the detailed system design or architecture. As noted in chapter 3, five defense contractors have competed with different BMD system designs. The Strategic Defense Initiative Organization (SDIO) has conducted additional analyses outside the main architecture contracting framework. A single system architect is to be chosen in This architect is to define the actual BMD system in detail, providing information for a decision on whether to proceed to the next step: full-scale engineering development. The SDIO has pro- Note: Complete definitions of acronyms and initialisms are listed in Append-x B of this report. posed an early 1990s decision on FSED but its schedules are slipping as a result of funding levels that are below its earlier expectations. In the meantime, common elements in the existing architecture studies can be used to guide the research program. All of the spaceand ground-based architecture designs included space-based infrared (IR) sensors and space-based interceptors (SBIs). All assumed 1 Each architect defined three architectures: a combination space and ground-based system, a ground-only system, and a theater defense system. In addition, most architects have considered various time-phased options. For this discussion we are considering primarily the combined space- and ground-based architectures. 159

167 160 some type of ground-based exe-atmospheric The concept validation program approved reentry interceptor system (E RI S). All saw a by the Secretary of Defense in September 1987, critical need for midcourse interactive discrimi- included work on SBIs, ERIS, and associated nation, although this task might be too diffi- sensors and battle management technology. cult for a near-term, phase-one deployment. The system engineer must combine various components and sub-systems defined by the architecture into a working system. A typical BMD system as envisioned by system architecture contractors for intermediate-term ( phase-two ) deployment might have included 30,000 major sub-systems of nine different types (for suggested major components of a phase-two system, see table 1-2 inch. 1). The sub-systems would be tied together by a communications network. These sub-systems would have to work together under the direction of battle management computers. 2 For each of these components, the system engineer would have to consider the following issues: Mass is particularly critical for SBIs: they would have to be light to reduce space transportation costs and to achieve the necessary velocity during battle. Total volume may be limited by the space transportation system. All space sub-systems would have to conform to the launch vehicle internal dimensions, preferably with minimum wasted payload space. For early deployment (late 1990s), the choices for space base-load power would be limited to solar (which is vulnerable), or nuclear, which would have to be developed and space-qualified in the power ranges needed for BMD. Far-term directed-energy weapons could be driven by liquid oxygerd/liquid hydrogen turbogenerators or fuel cells for a few hundred seconds. The weight of power supplies might dominate future systems. As discussed inch 7, this battle management function would likely be distributed among many computers on different satellites for survivability. SYSTEM DEVELOPMENT Heat rejected by the various devices would have to be minimized and properly managed, since cooling systems take up weight and power. Almost all sub-systems would have to be produced in large quantities compared to previous space systems. These components would have to be capable of mass production, as compared to the one-of-akind laboratory fabrication used in many of the SDIO technology demonstration projects. The United States has never mass-produced any satellites. All components would have to withstand severe radiation environments, including nearby nuclear explosions. This would be particularly stressing on electronic components such as IR detectors. The detectors and most electronics used for demonstration experiments would not be suitable for BMD deployment. These systems would have to endure and operate on call after sitting dormant (except possibly for periodic tests) for years. The current goal is at least 5-year life for first-phase deployment, with 7 years desirable. Limited lifetimes would further burden the space transportation system with replacement or repair missions. Many systems might have to operate within seconds or minutes after warning, although there might be an alert status lasting for days or weeks. Trade-offs between long alert times and fuel consumption might be necessary. All space-based systems would have to operate automatically, compared to the careful hand tweaking common in experiments. In particular, there would be little or no opportunity for the routine

168 161 maintenance common to all terrestrial military systems. Various sub-systems and components would have to work together. For example, radiation from a nuclear power supply must not degrade the operation of sensitive IR sensors or electronics. Similarly, fumes from a propulsion system must not fog the optics of critical sensors, and vibration from power sources must not degrade weapons pointing accuracy. If components are prone to failure, they should be easily replaceable or adjustable. For space-based systems, a key issue would be whether to replace entire satellites when they failed, or to attempt periodic manual or robotic repair. All systems and components should survive both natural and man-made environments. Survivability measures such as decoys, redundancy, shielding, maneuver- ability, electronic jamming, and shootback would add mass to space-based components. One system architect estimated that survivability measures would account for 70 percent of on-orbit mass for SBI systems. The communications channels would have to be secure against interception, manipulation, and jamming. The systems should be safe in manufacture, assembly, transport, and operation. SDIO is funding research in all of these areas. Optimists believe these characteristics may be achievable; pessimists question whether the break necessary from past practice and experience is possible; others say it is too early in the research program to judge whether the United States could achieve all of these attributes in a working system. SYSTEM Testing of both hardware and software is essential to any engineering project. Components are tested and modified to overcome deficiencies. Sub-systems are tested and modified. Finally, prototypes of the complete system are built and tested under full operating conditions whenever possible. These system tests invariably reveal faults in the original design, faults which must be corrected before production begins. A ballistic missile defense system could not be tested in a full battle condition. Instead, the systems engineer would have to rely on some combination of computer simulations and operation under simulated conditions. The anti-ballistic missile (ABM) treaty prohibits space-based tests in an ABM mode which would be necessary to establish even minimal confidence in SBIs. In place of complete system testing, the SDIO is developing the National Test Bed. This test bed (see ch. 8) is to tie together many TESTING communication nodes and computers via satellite, simulating some of the complexity of BMD. Some types of hardware (such as sensors) would also be coupled into this test system as they became available, talking to the computers as they would in a real battle. The cost of simulation will be high, but this is the only way to give leaders some degree of confidence in system operation. One of the key judgments the President and Congress will have to make about the SDI program will be the level of confidence to be placed in a global system that has never been tested in a full operational mode. Testing so far under the SDIO program has been limited to the component or sub-system level, usually under simplified or artificial conditions. These experiments have yielded valuable information necessary for the ongoing research and development effort; the United States should not, however, confuse a demonstration test with operational readiness (see box 6-A).

169 162 Box 6-A. SDIO Demonstration Experimental Homing Overlay Experiment: The HOE demonstrated on the fourth test (June 10, 1984) that an experimental IR homing vehicle can acquire and collide with a simulated reentry vehicle in flight. The RV was launched aboard a test ICBM from Vandenberg AFB in California. After detection by radars on Kwajalein, a rocket carrying the experimental ground-launched interceptor was fired from a nearby island toward the oncoming RV. The IR sensor on the interceptor then acquired the RV and guided the interceptor to a direct hit high above the Pacific. While this was an encouraging and successful experiment, it does not mean that the United States could deploy operational exoatmospheric interceptors tomorrow. The HOE experiment used parts of an existing missile, too large and expensive for an affordable BMD system. The IR sensor was cooled for many hours prior to the test; an operational system could not be maintained at such cold temperatures. The detectors were not hardened against nuclear radiation; new types of detectors would be required for the operational system. The simulated RV fired from Vandenberg AFB in California radiated about 10 times more IR energy than that expected from today s Soviet RV, and future RVs could have even lower IR signatures with thermal shrouds. There was only one RV, and the experimenters knew when and where it would be fired; the real issue for exoatmospheric interception is decoy discrimination-separating one RV out of a cloud of hundreds or thousands of other objects, including tethered balloons. Opinions differ on how difficult this would be. Delta 180: The Delta 180 mission (Sept. 5, 1986) launched a Delta missile into space; the two upper stages of this missile were both placed in orbit. Each contained sensors later used to measure radiation from the other and from another missile launched from White Sands, New Mexico during one orbit. One stage also contained a radar sensor used to guide the two stages into a collision course at the end of the experiment. The Delta 180 was a very successful measurement program, providing useful information about radiation from rocket exhaust plumes, both at close range in space and from the ground-launched Aries rocket. Some radiation patterns confirmed expectations, but there were some surprises which could improve our ability to detect and track future missile plumes. Tracking algorithms were also tested in the final interception with the target stage accelerating, which is more difficult than for targets with constant velocity. The entire Delta 180 mission took only 18 months from start to finish, requiring extraordinary management and dedicated performance by defense contractors. However, this measurements program should not be confused with a demonstration of the near operational readiness of space-based interceptors. This interception had little resemblance to the BMD problem and could not have without violating the ABM treaty. The relative velocities and ranges of the two stages were far less than those required for BMD. The target stage had a large radar reflector (over 1 square meter). The size and mass of the interceptor stage (over 2,000 kg compared to a goal of less than 200 kg for SBIs) would eliminate any possibility of achieving the velocities required of a SBI to kill an ICBM. All planned SBIs discussed to date would require an IR sensor for final homing, while Delta 180 used a Phoenix air-to-air missile radar. Finally, the near head-on aspect of the final kill would not be typical for a BMD mission, and did not stress the divert capability of the interceptor. FLAGE: Six of nine planned tests of the flexible, light-weight agile guided experiment ( FLAGE) short-range terminal interceptor missile have been completed. On the second test, the radar-guided homing interceptor passed very close to the target, again indicating that hit-to-kill interceptors are feasible under appropriate conditions. In the FLAGE tests, the target vehicle was flown into a highly instrumented volume of air above the White Sands Missile Range. Although artificial, this controlled environment is appropriate for an experiment, which should collect as much data as possible. The successful interception These comments on the SDI validation experiments should not be construed as criticism of SDIO management. These are all sound experiments properly designed to collect bits of information necessary on the path to developing a working system. At this time we have no major element of a non-nuclear ballistic missile defense system which has been tested in a system mode with equipment suitable for actual operation.

170 163 does not imply that the United States could build a FLAGE interceptor system today that would be effective against uncooperative targets in all types of weather. A FLAGE-derived interceptor would not be suitable for defending soft targets such as cities. MIRACL Laser Test: The MIRACL DF laser at White Sands was aimed at a strapped-down Titan rocket casing. The booster casing was stressed with high pressure nitrogen to simulate the stresses expected in flight. The laser beam heated the skin of the tank, which then exploded in a few seconds as the shell weakened. This experiment essentially tested target lethality: how much IR energy is required to weaken a Titan tank until it ruptures? The laser beam was about 100,000 times less bright than one required to destroy a responsive Soviet booster from a distance of 1,000 km or more. It was not a test of a directed-energy weapon system. The key issues for any DEW are target acquisition and tracking, beam pointing over very large distances, and particularly the questions of retargeting and beam jitter: could one keep the laser beam focused on one spot on the booster body while the booster and the DEW platform travel through space at many kilometers per second? Other more complex experiments would be required to answer these crucial questions. Real confidence in any DEW would require space-based testing under dynamic conditions. Once a system had been developed and tested to the degree possible, it would have to be manufactured. The manufacturing tools and facilities to fabricate much of the specialized equipment needed for BMD are not yet available. In some cases, expansion or modification of existing manufacturing facilities might be adequate. In other cases, entirely new manufacturing techniques would have to be developed and skilled workers trained. The SDI research program is addressing some key manufacturing issues, such as mirror and focal plane array (FPA) fabrication techniques. Some of the key manufacturing challenges are summarized in table 6-1, along with an estimated comparison of current manufacturing capacity with second-phase BMD needs. These comparisons are not always valid, however. For example, current (FPA) manufacturing capacity is for non-radiation hardened arrays with less than 180 detector elements. Ballistic missile defense sensors must survive in a radiation environment, so new types of detectors are being developed, along with all new manufacturing techniques. The items in table 6-1 represent only phasetwo BMD deployments, excluding items such as interactive discrimination apparatus and FABRICATION Table 6-1. Examples of Current v. Required Manufacturing Capacity for Proposed BMD Systems Current capacity Required capacity for Phase-n BMD Large area mirrors (square meters per year) ,000 Focal plane arrays (number of elements made per year) Sapphire windows (for HEDI; number per year) ,000 Precision guided missiles (per year) loos 1,000-5,000 Satellites (per year) s Space-launch rockets s loos SOURCE: Office of Technology Assessment, 1988, directed-energy weapons (DEW). 3 Building hundreds of space-qualified neutral particle beam accelerators or high power lasers with their rapid pointing and retargeting mechanisms would certainly stress manufacturing capability. Any manufacturing process must minimize cost and delivery time while maintaining high quality. These three virtues have added significance for BMD. Note, however, that recently the SDIO has suggested the possibility of including such elements in phase two.

171 164 Photo credit: U.S. Department of Defense Delta 180 payload The payload of the Delta 180 experiment, atop a Delta booster, is shown during shroud installation on Pad 17 at Cape Canaveral. Multiple boxes carrying optical sensors are mounted on the side of the rocket s second-stage truss at bottom. The mast on top of the third stage is a Phoenix missile sensor, which helped guide an intercept between the two vehicles to obtain rocket motor plume data at short distances.., Photo credit: U.S. Department of Defense Photo credit: U.S. Department of Defense Lethal test of high-velocity projectiles. Electromagnetic launchers might hurl small homing projectiles at distant missile stages or warheads. In this test of the effects of high-velocity impact, a small (unguided) plastic projectile hit a cast aluminum block at 7 km/s. This was a test of lethality, not of a weapon: the projectile was not launched from an electromagnetic launcher. Laser lethality test. In September 1986 this test at White Sands Missile Range, N. M., investigated the possible effects of a laser beam on a rocket booster. The test vehicle was the second stage of a Titan I booster missile body. External loads were applied to the booster to simulate flight conditions typical of current operational Soviet missile systems. The test vehicle contained no liquid propellant or explosives. It was irradiated with a high-energy laser beam for several seconds before being destroyed. The laser used, the Mid-Infrared Advanced Chemical Laser, generates a beam energy greater than 1 MW/sr. It is a test laser, not developed for deployment in space.

172 165 cost The projected costs for a BMD system will strongly affect a national decision on whether to proceed with production or deployment. In addition to total costs, the incremental costs of BMD would have to be less (some think substantially less) than the perceived incremental cost of Soviet countermeasures. Thus the unit costs of a deployed SBI might have to be less than l/370th to l/12th the cost of a Soviet booster. On the other hand, the leverage provided by a successful adaptive preferential defense might improve this cost-exchange ratio (see ch. 1). The allowable costs for aground-based exoatmospheric interceptor would depend on the system architecture. With low leakage from the boost phase and good discrimination, each interceptor would have to engage only a small percentage of the attacking Soviet reentry vehicles (RVs), and the interceptor could be relatively expensive. If discrimination were poor, which might be the case in a phase-one deployment, then the interceptor might be competing with cheap decoys. The defense would not be cost-effective at the margin if every exoatmospheric interceptor had to costless than 10 light-weight decoys, or even less than 10 heavy decoys. 5 Time The time to manufacture components for BMD might be crucial in several respects. The 12-tOl cost ratio assumes that 8 percent of the SBIS would be within range of the Soviet missile fields and that one SBI is fired at each booster or PBV. Them are no extra SBIS for redundancy or shoot-back against Soviet ASATS. In this case the United States would have to add about 12 SBIS (and another carrier satellite) for each new Soviet booster. The 370- t~l cost ratio comes from a concentrated basing of new Soviet boosters in a relatively small area say 150 km by 150 km. In this case the United States would have to deploy 370 extra SBIS and their associated satellites for each new Soviet booster to achieve an 85 percent probability of destroying that extra RV. If the boost phase defense let through 10 percent of the boosters, and each booster carried 10 warheads, 10 heavy decoys, and 100 light decoys, then the exe-atmospheric interceptor system would have to engage one warhead, one heavy decoy, and 10 light decoys for each booster launched With perfect discrirnin ation one deployed interceptor would have to cost less than one loaded booster. Without any discrimination, one interceptor would have to cost l/12th of the booster. Ideally the system should be deployed quickly to avoid transition instabilities, although system architects differ on this point. Components could be produced and stockpiled until deployment began. 6 To the degree that space transportation would pace deployment, production times would not be critical. But the United States would also be locked in a race with Soviet countermeasures. If the United States could not produce and deploy enough SBIsbefore the Soviets had reduced a substantial number of their booster burn times below 140 seconds, then BMD boostphase system effectiveness would drop significantly, perhaps to zero. The SBIs might force the Soviets to faster post-boost vehicle (PBV) dispersals, which could reduce the number of RVs At some point, however, there would be no sense in deploying SBIs(and particularly SBIs Which did not have any midcourse capability against RVs) until DEW were developed. (See also ch. 5 and the key-issues section at the end of this chapter for more analysis of SBI effectiveness against boosters with moderately fast burn times.) On the other hand, if the United States could produce and deploy an SBI system in a few years, and if it could build and deploy a credible DEW system as the Soviet Union converted to faster-burning boosters and fast-dispersing PBVs, then BMD effectiveness might continue.. Production time involves not only the production rate, but the time to design, build, and debug the manufacturing facilities, including necessary training of production workers. Since many new technologies are contemplated, there might be relatively long periods before routine production could begin. Quality Quality control would be essential, particularly for space-based deployment. Repair or even replacement of failed assets in space See U.S. Congress, Office of Technology Assessment, Ballistic Missile Defense Technologies, OTA-ISC-254 (Washington, D. C.: U.S. Government Printing Office), p. 119.

173 166 might severely stress space transportation, particularly if space launch facilities were completely occupied over a period of 5 to 10 years just to lift the initial BMD equipment into place. Given that some boost-phase defense capability would be key to a highly effective BMD system, and given that the United States currently has very little space launch capability, deployment of space-based assets would most likely limit the operational starting date for BMD. As shown in the space transportation section of chapter 5, the United States would have to build a new space launch system to lift into orbit the necessary number of SBIs and their supporting satellites. The timing of the development and availability of a new space launch system is unclear, but it is doubtful that it would be possible to launch significant numbers of SBIs before the mid to late 1990s. DEPLOYMENT Several years of continuous space launch activities from several launch pads would then be necessary to deploy enough SBIs to provide one shot against each missile or PBV in today s fleet of Soviet intercontinental ballistic missiles (ICBMs). The SDIO, however, does not propose deploying that many SBIs in a firstphase system. It argues that lesser capabilities would still have worthwhile deterrent value. (See section below on scheduling and deployment issues for discussion of the effect of deployment rates on SBI system effectiveness.) OPERATION AND MAINTENANCE Once deployed, the BMD system would have to be kept in operating order. Ground-based elements such as ERIS could be periodically tested, disassembled, and repaired as needed. For space-based assets, both testing and repair would be difficult unless built into the initial design. Methods would be needed to determine if the sensor or the guidance system on a dormant SBI would operate in a war. Computer systems would have to be exercised to make sure radiation in space had not altered a key software bit that might subsequently inhibit successful operation. The status of dormant space assets would have to be monitored carefully and frequently. Once defective space systems were diagnosed, they would have to be replaced or repaired. The system architecture would have to incorporate some combination of redundancy or on-orbit repair or replacement to maintain the total system. The space transportation system would have to be sized to handle this load. Space-based assets might also need to be modified in response to Soviet countermeasures. SBI sensors initially designed for tracking only booster plumes with short or mediumwave IR sensors might become worthless against faster-burning boosters. Should a second-phase system add LWIR sensors to previously deployed SBIs to give them midcourse kill capability? Trade-off studies would determine whether it would be more cost-effective to replace components on obsolete satellites or simply to add entirely new satellites.

174 167 EXAMPLE BMD SUB-SYSTEM: SSTS To appreciate some of the complexity of a BMD system, consider just one of the systems in table 1-2: a moderately sophisticated Space Surveillance and Tracking System (SSTS). The potential sub-systems of an SSTS are shown in table 6-2. Almost every subsystem on this list would require development to meet the probable BMD specifications. At the next level down, just one sub-system from the SSTS, a three-color LWIR sensor, Table 6-2. SSTS Subsystems Propulsion (for station-keeping) Communications (space-to-space) Communications (space-to-ground) Power source Three-color LWIR sensor(s) SWIR/MWIR sensor(s) Laser ranger/designator Star tracker(s)., Computer and memory Waste heat rejection Support structure Development required Low High High High High Medium High Medium Medium Medium Low would include the components listed in table 6-3. Again, most of these components must be developed to meet BMD specifications. An analysis of the other SSTS sub-systems and the other major sub-systems in the three phases of SDI would reveal literally hundreds of sizeable development programs which would have to come together to form the complete system. Table 6.3. Three-Color LWIR Sensor Components Primary mirror Secondary mirror Cryo-cooler Three-color focal plane array (FPA) Signal processor Three-axis gimbal Servo control system Thermal control system Sun shield Support structures SOURCE: Office of Technology Assessment, 1988 SOURCE: Office of Technology Assessment, Development required High High Medium High High Medium Medium Low Low Low Building and deploying a system on the scale of proposed BMD architectures would stress the U.S. engineering and manufacturing infrastructure on many fronts. However, three critical systems issues are unique to ballistic missile defense with space-based components: the lack of realistic system testing, the necessity for automated, computer-controlled operation, and the difficulties of scheduling and space deployment. System Testing The inability to test fully a global BMD system (both hardware and software) would cast KEY SYSTEM ISSUES doubt on its operational effectiveness. The administration and Congress will have to decide on the deployment of a system whose performance would have to be predicted largely by computer simulations. The National Test Bed and future component tests would improve the verisimilitude of those simulations, but they could not encompass all of the complexity of the real world. Some issues such as sensor operation against a nuclear explosion background in space could not be tested even at the component level without abandoning the Limited Test Ban Treaty. Except in computer simulations, the system could not be tested, short of war, with even 10 percent of the possible wartime threat.

175 168 It is true that all military systems are subject to uncertainty when they first go into battle. A fighter aircraft, despite the best flight test program, can never be tested with all the variables that will arise in a real battle. An aircraft-carrier battle group could never anticipate all possible situations in some future battle with a capable adversary, and might be susceptible to unforeseen vulnerabilities. The U.S. carrier battle groups have never fought against an enemy with modem smart weapons. There is uncertainty in the performance predictions of these conventional military systems. A global BMD system would have even more complex, untestable sub-system interactions. Even full interception tests, using SBIs fired against ICBMs launched from Vandenberg AFB, could involve at best a salvo launch of a few missiles. This would not substitute for the launching of a thousand missiles by the Soviet Union at a time of their choosing, preceded by anti-satellite weapon (ASAT) attacks and nuclear precursor explosions. Individual components such as sensors, data processors, and communication equipment could be tested by themselves to full operational capacity in the laboratory or in simulated space chambers, and some effects of nuclear explosions could be tested at the Nevada test site. In any case, the complete BMD system could not be tested as an integrated unit against a real threat. Neither, on the other hand, could the Soviet offensive ballistic missile force. Automatic Operation Automation has made dramatic changes in factories and some military weapons systems. Robotics is firmly established in many manufacturing situations, and will grow in the future. However, space-based BMD systems would cross into new engineering domains of automatic operation on several counts: continuous unattended stand-by operation for years, a continuously changing constellation of components which would have to operate together as a unit, and operation under adverse conditions against an opponent determined to defeat the system. None of these limitations is encountered in automated factories. Automatic fire control systems are common in today s weapons. Human intervention is always possible, however, to repair and maintain the system. The United States has never operated a weapon system in space. Both the United States and the Soviet Union have operated sensor systems ins ace for surveillance and early warning of ballistic missile attack. The challenge would be to integrate more sophisticated early warning satellites with actual weapon platforms thousands of kilometers away. Sensor satellites currently in orbit operate autonomously, with directions from a few ground-based mission control nodes. Once the battle began, BMD systems might require the autonomous operation of 30 to 40 sensors working in conjunction with hundreds or thousands of SBI carrier satellites. Sensors and carrier satellites would be moving in different orbits, so that the particular weapons platforms and sensors making up a battle group (in one possible battle management architecture) would be constantly changing with time. (See ch. 7 on wartime operation.) These battle groups would have to be connected by secure communication links. Higher system effectiveness would entail tighter coordination. Automatic operation would be further challenged by Soviet defense suppression tactics. The system would ideally adapt to lost or noisy communication links and continue to manage the battle on the basis of degraded information. (See ch. 9 for a fuller discussion of BMD software dependability.)

176 169 Scheduling and Deployment: An Illustrative Scenario If an administration and Congress were to decide that our national security would be improved by deploying some type of BMD, a major issue would be when to begin deployment. Early deployment (e.g., ) of a phaseone system would risk locking in immature BMD technology that might be less effective against the projected threat. Waiting for more advanced technology would give the Soviet Union more time to prepare countermeasures, increasing the risk that the defense effectiveness would remain low. Early deployment would strain space transportation facilities, and the long deployment time would preclude a fast transition from offense- to defensedominated status. But a decision to wait for later deployment could, some fear, indefinitely postpone any deployment at all. Ballistic missile defense system effectiveness would depend not only on the U.S. deployment schedule, but also on the timing of Soviet countermeasures. The longer it took to deploy a defense, the more time the Soviets would have to respond by improving their offensive forces. To illustrate the interplay between defensive and offensive deployments over time, OTA constructed a plausible scenario for the period, then estimated the effectiveness of an SBI system as a function of time. For the defense, we assume that: SBI deployment would be limited only by the capacity of future United States space transportation systems. That is, the United States could produce and operate in space as many SBIs as it could launch. Note that it is emphatically not the SDIO proposal to deploy this many SBIs. The SDIO two-track space transportation scenario succeeds in building a heavy lift expendable launch vehicle by 1994 with 30,000 kg lift to near polar orbits, and this same technology simultaneously evolves into an economical, partially reusable vehicle with 44,000 kg capacity by the year Three new launch pad complexes would be built at Vandenberg AFB and launch rates would be increased from 3 per year per pad up to 12 per pad per year, bringing the total lift capacity to near polar orbits to 2.2 million kg per year by Three different classes of SBIs might be available with varying masses and velocities: a state-of-the-art a realistic, and an optimistic interceptor. (Specification of the characteristics of each are in the classified version of this report.) The SBIs would be replaced at the end of a useful life of 5 to 1O-years, which limits the number of SBIs in orbit unless the space transportation system capacity continues to grow with time. For the Soviet offensive response, OTA assumed: a gradual decrease in the burn-time of Soviet ICBM boosters and in the RV and decoy dispersal time of its PBVs through the introduction of one new class of 10- warhead missiles every five years; that these new missiles would be clustered at three existing SS-18 missile sites, which would cover an area of 500,000 square km; retirement of old Soviet missiles as the latest models were introduced, keeping the total RV count at 10,000 (case 1), or an increase of their ICBM s by 100 per year after the year 2000 (case 2); no other Soviet countermeasures, except a significant Soviet A SAT capability, imlied by our resewing a substantial fraction of U.S. SBIs for self-defense or to account for inoperable SBIs that fail over time. While these assumptions are technically plausible, they are not based on any Department of Defense or intelligence community estimates of what the Soviets could or would do.

177 170 Table 6-4. Assumed Distribution of Soviet ICBMs for All ICBMs are assumed to carry 10 warheads. Please note that the mix of forces here reflects neither Department of Defense nor intelligence community estimates of what the Soviets actually may do. Instead, this table merely lays out a purely hypothetical sequence of a phasing-in of fasterburning ICBMs at 5-year internals beginning in 1990, Older missiles are retired as new ones are deployed, keeping the total RV count fixed at a hypothetical number of 10,000. The slow-burn boosters are distributed over existing Soviet missile fields, while the other four classes are assumed concentrated at three existing sites. Number of ICBMs ICBM type SBB MBB-1 MBB-2 FBB-1 FBB-2 Year: Legend: SBB: Slow-Burn Booster MBB-1: Medium-Burn Booster First Generation MBB-2: Medium-Burn Booster Second Generation FBB-1: Fast-Burn Booster-First Generation FBB-2: Fast-Burn Booster-Second Generation SOURCE: Office of Technology Assessment, 1988 Space Transportation Limits on Deployment As indicated in chapter 5, a new space transportation system would be needed to launch the space-based assets of a highly effective BMD system. Even a more modest system, such as that proposed by SDIO for the first phase, would call for considerable new space transportation capacity. The SDIO has identified two potentially conflicting space transportation goals: reducing launch costs by a factor of 10 and beginning some launches in the mid-1990s. Derivatives of existing Shuttle/ Titan launch systems are not likely to lead to major cost reductions; an entirely new system would be needed. But a revolutionary new space transportation system would not likely be ready before the year To achieve both the cost and schedule goals, SDIO has proposed a dual-track formula: a new space transportation system would be developed with a goal of a tenfold cost reduction by 2000 or so, but parts of this new system would be available by the mid-1990s for early deployments, probably with reduced lift capacity and higher cost. This approach might create design compromises. Either cost reductions might have to be postponed to meet the schedule, or the schedule might have to be slipped to meet the eventual cost goals: a space transportation system designed to meet just one of these goals might look quite different from the hybrid. In this scenario, however, we assume that both goals could be achieved simultaneously. The United States now has one pad capable of launching more than 10,000 kg to the high inclination orbits and altitudes of several hundreds of kilometers to be occupied by the SBI constellation. 7 The Shuttle pad at Vandenberg Air Force Base could be modified by 1992 to launch the Titan-4 (CELV) vehicle with a capacity of about 14,500 kg to SBI orbits. In the past, building new launch pads has taken from 7 to 10 years and there is some question whether there is adequate space at Vandenberg to add even a few more pads and their necessary assembly facilities. (The Air Force has been examining the possibility of launching rockets from an off-shore oil rig.) Survivability of launch facilities would also be questionable if all U.S. polar-orbit pads were located at one coastal site. In this scenario, we assume that these difficulties are overcome. Launch rates have been in the range of three to five per year from one pad. This rate is The 4-East pad at Vandenberg Air Force Base in California is equipped to launch the Titan 34D and Titan-4 (CELV) vehicles mto polar or high inclination orbits. The 4-West pad at Vandenberg can handle tlw Titan-2 vehicle, which has less than 2,000 kg capacity. Two pads at Kennedy Space Center (#40 and #41) can launch Titan 34Ds and Titan-4s, but not into near-polar orbits. There are no Delta launch facilities at Vandenberg.

178 171 limited by the necessity to assemble the launch vehicles at the site. Studies are underway to determine if these launch rates could be increased to 6 per year, with some experts suggesting that rates up to 8 per year might be feasible in the future for the Titan class vehicles, and 12 per year per pad for the new vehicle. SBI Characteristics OTA analyzed three classes of SBIs, corresponding to assumed improvements in SBI technology as discussed in chapter 5. The state-of-the-art rocket would probably be the best technology available for a first-phase deployment in the mid-1990s. For the most part, this SBI would use components that have been demonstrated in the laboratory (as of 1988), but not as yet assembled into a working system. The realistic SBI represents a plausible level of technology after more component research and development, and might be available by the mid to late 1990s; the overall rocket mass assumption of well under 100 kg would be particularly challenging. The optimistic SBI assumes improvements in all areas of development, and would be much less likely, but possible. Other assumptions about SBI redundancy factors and kill probabilities are the same as those applied earlier in chapter 5 of this report. Given the optimistic space launch projections from chapter 5 and the different assumptions for SBI masses, one can estimate the total number of SBIs that might be placed in orbit as a function of time, as shown in figure 6-1. The lifetimes of SBIs in space would be critical, since defunct interceptors would have to be replaced, taking space transportation capacity away from the tasks of increasing SBI deployments or other BMD assets. It might turn out, however, that on-orbit repair could reduce the numbers of spares and replacements needed. As shown in figure 6-1, the number of state-of-the-art rockets would reach a plateau by about 2006 if better SBIs could not be developed: a space transportation system sized to put the original constellation in place would operate full-time just to replace these I Year Maximum number of SBIs that could be launched into orbit based on the assumed space transportation revolution described in chapter 5. This chart assumes that all space launch capability is devoted to SBIs and their associated carrier vehicles. The net mass per SBI, including the pro-rated share of the carrier vehicle mass, would be 334, 179, and 129 kilograms for state-of-the-art, realistic, and optimistic SBIs, with life-times of 5, 7, and 10 years. SOURCE: Office of Technology Assessment, SBIs and maintain the constellation in a steady-state constellation. For the lighter and faster optimistic SBIs, the assumed transportation system could lift up to 160,000 SBIs into orbit by the year This assumes that no other space assets would be launched into near polar orbits during the entire period. Thus any later deployments of interactive discrimination systems or directed-energy weapons would reduce the possible number of SBIs in orbit. In any case, it is obvious that the United States would not try to manufacture, lift into space, and manage a constellation of 160,000 SBIs. The optimistic SBI effectiveness curves which follow are therefore unrealistic; they are shown only to indicate upper bounds on SBI boost and post-boost effectiveness. They suggest that while SBIs might be considered for a system intended to enhance deterrence, they would not, by themselves, be suitable for a system intended to assure very RV low leakage rates. They also suggest that, barring substan-

179 172 tial offensive force reductions, the initial effectiveness of an SBI system might be eroded by appropriate countermeasures. In that case, directed-energy weapons might have to be brought on line just to maintain previous defense capability. Assumed Soviet Offensive Countermeasures As the U.S. space transportation system (and hence the number of possible SBIs in space) grew, Soviet ICBM and submarinelaunched ballistic missile SLBM forces would most likely also change with time. One central question for evaluating BMD effectiveness is whether reasonable Soviet countermeasures could keep ahead of possible U.S. BMD deployments. Here, OTA analyzed the effects of just three Soviet countermeasures: reduced booster burn and PBV dispersal times and clustering of new missiles at three existing missile sites. These analyses assumed that the Soviet Union reduced its booster burn and PBV times gradually over the next two decades, introducing anew class of weapon each 5 years with moderately improved performance. Three cases were assumed: optimistic (relatively long booster bum times), base case, and pessimistic threats. Even the pessimistic threat case assumes a 90-second bum-time by 2006, still more than the 60- to 80-second burn-times deemed feasible for the next century by some rocket experts. Thus these threat assumptions are all conservative compared to what may be technically feasible. SBI Boost and Post-Boost Effectiveness We next calculated the maximum possible number of RVs that could be destroyed each year by SBIs in either the boost or the postboost phase, simply by calculating how many SBIs would be within range of the booster or the post-boost vehicle at the time each RV was deployed. We assumed uniform, serial RV deployment over the PBV dispersal time. Each SBI attacked the booster first if it was within range, then the PBV at the earliest possible time. Two shots were taken if more than one SBI could reach a booster or PBV. Perfect battle management was assumed: the battle manager knew exactly where all boosters and SBIs would be at burnout, and assigned SBIs to their highest value targets without error. These calculations assumed that a substantial fraction of SBIs are used for self-defense (or are inoperable) an on orbit repair system, however, might reduce the extra numbers needed. Other assumptions were that each SBI had a reasonably high single-shot kill probability against the boosters and and a slightly smaller one against the PBV. The resulting system effectiveness (the number of RVs leaking through the boost and postboost SBI defense) is plotted as a function of time in figure 6-2 for the three canonical SBIs Figure 6-2.-Number of Warheads Leaking Through Boost and Peat-Booat Defenses Year BMD system effectiveness in terms of the number of RVs (out of a hypothetical attack of 1,000 missiles with 10,000 RVs) which would leak through a boost and post-boost defense, limited only by the ability of the U.S. space transportation system to lift space-based interceptors (SBIs) into orbit (figure 6-1 indicates the number of SBIs available each year for each type of SBI). The SBIs have a reasonable probability y of destroying a booster and a slightly smaller probability y of killing a PBV; a substantial percentage of the SBIs are used for self-defense or are otherwise inoperative. The Soviet threat has a constant 10,000 warhead level, but with decreases in booster burn-times and PBV dispersal times as described in the text. SOURCE: Office of Technology Assessment, 1988,

180 173 of chapter 5, assuming OTA s hypothetical Soviet threat. In the very near term, the United States could only deploy the state-of-the-art SBI. According to these simplified calculations, this type of defense could at best destroy 2,500 RVs out of the OTA-postulated 1,000- missile, 10,000-RV threat by 1998 when the United States would have orbited 4,100 SBIs; 7,500 RVs (and their associated decoys) would pass through to the later defensive layers. Performance would degrade over time with quicker dispersal of Soviet RVs. If the United States could develop the lighter and faster optimistic SBIs, then the defense could reach 50 percent effectiveness by 2001, but this would imply the deployment of 40,000 SBIs by then. Furthermore, to maintain this approximate level of effectiveness with 5,000 warheads leaking through to the midcourse, the United States would have to continue deploying these SBIs, reaching levels of 160,000 SBIs by Even then, the Soviet penetration to the midcourse would have increased slightly to 6,000 warheads. The most likely realistic SBI would result in a minimum leakage of 6,000 warheads to midcourse. To come close to maintaining this leakage, the United States would have to continue devoting all space launch capability to the SBI system; by 2010 there would be 90,000 SBIs in orbit and 8,000 warheads would survive to midcourse. Again, such figures illustrate that SBIs should not be expected to stop high percentages of Soviet missiles in a massive attack. Nor is it reasonable to expect them to sustain initial boost- and post-boost phase capabilities against a responsive Soviet missile threat of the future. The SDIO does not support either expectation. The sensitivity of SBI effectiveness to the Soviet threat is shown in figure 6-3, assuming the realistic SBI rocket parameters in all cases. With the optimistic threat scenario, the SBI BMD system could achieve 50 percent effectiveness by 2005, assuming that the United States had deployed 70,000 SBIs. Again, this constellation would have to be increased to 90,000 by the year 2010, and even then the Soviet RVs leaking through could Figure &3. -Number of Warheads Leaking Through Boost and Post-Boost Defenses 10, A 4 Optimistic threat J I 3 l-- I 2 1 I I I (Realistic SBl) Year Boost and post-boost system effectiveness as a function of time for three different Soviet threat models described above. Realistic SBIs were used in all cases. SOURCE: Office of Technology Assessment, number 6,000 warheads and be increasing. These numbers suggest that directed-energy weapons would be needed, sooner or later, to achieve and sustain high kill levels against advanced Soviet boosters and PBVs. The previous two figures assume that the Soviets retire old missiles as new ones are deployed, keeping the total at 10,000 warheads available. In the absence of arms control treaties, they could keep old missiles in place, and continue to add faster-burning boosters. The BMD effectiveness for this situation is shown in figure 6-4, assuming that all initial mediumburn boosters are retained, and that 100 of the faster-burning boosters (FBB) are added each year after Under the most optimistic (for the defense) conditions, the Soviets could maintain 6,000 warheads surviving into mid-course. Assuming penetration aids to be available by the time period, these 6,000 warheads and their associated decoys would make passive midcourse discrimination and RV kills very difficult. The leakage against SBIs in all cases would increase with time, most likely reaching the 10,000 warhead level by 2010, despite the presence of up to 160,000 SBIs in space. 8 8 For analytic purposes, we have ignored the questions of maintenance and battle management of so many interceptors.

181 174 Figure 6-4b. -Number of Warheads Leaking Through Boost and Post-Boost Defenses - ~ Optimistic SBI 2 o~ I I I Year Boost and post-boost effectiveness limited only by space t ransportat ion capability, assuming that the Soviet threat increases in quality (shorter deployment times) and quantity (after 2000). Effectiveness shown for three different types of space-based interceptors against the base case Soviet threat. SOURCE: Office of Technology Assessment, 1988, 01 I I I Year Boost and post-boost effectiveness against three different Soviet threats, all assuming realistic space-based interceptors. SOURCE: Office of Technology Assessment, Testing If the United States abandoned or achieved modification of the ABM Treaty, it could test a limited constellation of SBIs against a few ICBM s launched from Vandenberg AFB. But this would not replicate the conditions of a massive, surprise launch of hundreds or thousands of ICBMs, ASATs, and nuclear precursors from the Soviet Union. A BMD system could not be tested against the real threat of up to thousands of ICBMs combined with defense suppression and nuclear precursors. However, neither could such a coordinated offensive attack be fully tested. Key elements, such as IR sensors, could not be realistically tested against a background disrupted by nuclear explosions without abrogating the Limited Test Ban Treaty. SYSTEM CONCLUSIONS Automation 4. No technical barriers appear to preclude automatic operation of a space-based BMD system, but the task of operating an automatic, constantly changing constellation of sensors and weapons platforms in the face of defense suppression tactics would be a major challenge with little or no analogous experience from any other automated systems. Phase One Scheduling and Deployment 5. A near-term deployment ( ) of state of-the-art SBIs might stop up to 2,500 of an assumed constant 10,000 Soviet warhead threat in the boost and post-boost phases if the United States devoted all of its space launch capability to lifting SBIs into orbit.

182 175 This assumes that the burn times and post-boost vehicle dispersal times of future Soviet ICBM s decrease over time in a reasonable manner. Of course, fewer SBIs could kill similar percentages of boosters if a smaller attack were assumed. The SDIO argues that defenses that are far from perfect still offer significant enhancement of deterrence (see chs. 1,2, and 3). Phase 3 SBIs and into midcourse after 2005, reaching the pre-bmd levels of 10,000 leaking warheads by Therefore, SBIs should not be expected to achieve the strategic goal of assured survival against nuclear attack by a Soviet missile force unconstrained by arms reductions and limitations. 8. A highly effective BMD system would require either very effective midcourse dis- Phase 2 6. An intermediate-term or phase-two deployment of more advanced SBIs might kill energy weapon (DEW) system, and prefercrimination or a very effective directed- up to 5,000 of the hypothesized fixed number of 10,000 Soviet RVs in the boost and by the most optimistic space transportaably both, since an SBI system, as limited post-boost phases, but only by orbiting from tion system, could never assure that fewer 90,000 to 160,000 SBIs. Therefore, the than 5,000 Soviet warheads and their associated decoys would leak through to the United States would be unlikely to rely on SBIs for continued boost-phase interception of advanced Soviet missiles. 9. As concluded in chapter 5, it is unlikely midcourse, 7. Given the assumptions of OTA analyses, that the United States could determine under the most optimistic conditions the Soviet Union could maintain an RV leakage 1990s, and deployment probably could not the feasibility of DEW systems by the late into midcourse at or above the 6,000 warhead level by increasing the number of therefore appears likely that the Soviet begin until at the earliest. It ICBMs deployed by 100 per year after the Union, unless constrained by offensive arms year Under any of the assumed conditions, the Soviet Union could increase tain leakage rates of a few thousand nuclear control agreements, would be able to main- the rate of warhead penetration against warheads until at least the period

183 Chapter 7 System Integration and Battle Management

184 CONTENTS Introduction Battle Management Conduct of the Battle Battle Management Functions Interactions Among the Phases System Performance and Interaction Opportunities for Human Intervention Supporting Functions Communications Recovery From Damage and Failures Complexity of Battle Management Battle Management Architecture Conclusions Box 7-A. 7-B. 7-c. Boxes Algorithms Centralization, Distribution, and Coupling Hierarchies and System Design , 195 Page Figure No Analytic Chart BMD System. of Battle Figure Management Functions in a Phase-One Page.196 Table Table No Ballistic Missile Battle Management Functions Page.181

185 Chapter 7 System Integration and Battle Management INTRODUCTION Chapter 6 discusses developing, deploying, and maintaining a ballistic missile defense (BMD) system. Once deployed, BMD components would have to work together to form a fighting system. Maintaining such integration would require regular, routine support. This chapter looks at integrated operation of the system. Although some system capabilities could be used during peacetime, e.g., for surveillance, fully integrated use would only be required during battle. 1 Accordingly, most of this chapter is concerned with battle management, i.e., how the system would be managed to fight effectively. Peacetime simulations of battles would also require considerable integration, but would probably omit operations such as use of interactive discriminators and firing and controlling weapons. A major assumption in the discussion that follows is that the system is sufficiently wellintegrated during peacetime that it can be moved promptly to a full fighting status. As examples, the communications network that permits battle managers to exchange information would have to be working and the battle managers would need timely data on the number, kinds, and locations of resources available to them. Peacetime activities needed to keep the system integrated, such as sending updates of resource-availability data and new versions of software to battle managers, would have to be performed routinely. Operational readiness, testing and evaluation, and repair or replacement off ailing system components would also have to be routine. BATTLE MANAGEMENT The battle management portion of a BMD system would combine the data provided by sensors and the capabilities offered by weapons into a defensive system. The battle management computers would provide computational and decisionmaking capability. The battle management software would be the glue bonding the components together into a fighting system. Battle management includes strategy, tactics, resource allocation algorithms, and status reporting. Battle management computing may be distributed among many different platforms or consolidated on just a few. In either case, the battle management functions would remain the same, although the capabilities needed in supporting functions, such as communications, Note: Complete definitions of acronyms and intialisms are listed in Appendix B of this report. might vary. This chapter describes the conduct of a battle from the battle management viewpoint, from alert through the actual battle sequence. The scenarios only meant to be illustrative, not comprehensive. Its purpose is to convey a sense of the complexity of the battle management task, not to provide an actual battle management system design. In peacetime, the system might be in a quiescent mode, conserving fuel and other resources, with some components shielded from space. As the probability of a battle increased, the system might move through a series of alert levels, during which sensors such as the Boost Surveillance and hacking System (BSTS) and Space Surveillance and Tracking System (SSTS) would be fully opened to space and weapons would be prepared for battle, including warm-up and status checks. At the highest alert level, the system would be fully prepared to fight a battle. 179

186 180 The battle may be partitioned into different phases, each distinguished by a different set of offensive actions. The phases are boost, postboost, mid-course, and terminal. For an individual reentry vehicle (RV) or decoy, the phases occur in the sequence given. Different RVs and decoys may be in different phases concurrently, requiring the defense to fight in different phases at the same time. In addition, the BMD system might have to defend itself against defense suppression attacks during any phase. For a description of the phases of ballistic missile flight, see chapter 3, table 3-6. CONDUCT OF THE BATTLE Our battle scenario assumes, for simplicity, a system with a second-phase architecture as described in chapter 3. We assume that it is in an alert stage from which it could be moved directly to fully automated battle management. 2 The battle would commence with the launch of Soviet intercontinental ballistic missiles (ICBMs). We assume a mix of ICBMs, some of which would burnout above the atmosphere, and some of which would bum out in the atmosphere. The ICBMs release post-boost vehicles (PBVs) carrying both RVs and decoys. The PBVs would maneuver to dispense their payloads, inserting each RV and decoy into a preplanned trajectory. RVs and decoys would then coast until they started to reenter the atmosphere. RVs would continue on to their targets, accompanied partway by those decoys designed to simulate reentry. Besides launching ICBMs, the Soviets might employ a variety of defense-suppression measures. For example, they might launch directascent anti-satellite weapons (ASATs) at BMD system satellites. Such a weapon might carry one or more warheads and decoys. The defense suppression attack might begin before an ICBM launch. The following sections describe briefly the functions that a BMD system would have to perform during the battle. Requirements for recovering from damage and failures occurring Although the assumption that there would be sufficient prior warning to an attack that a BMD system could be moved to an alert stage makes the scenario easier to describe, the system s designers could not depend on such an assumption to be true. There would have to be some provision to go from peace time to battle in seconds in the event that no warning is received or that such warnings are ignored. during battle are given simplified treatment later. Table 7-1 gives a more detailed description, with examples, of the defensive functions, organized by function and by missile flight phase. Battle Management Functions In all phases of a battle the defensive system would have to track targets, assign weapons to destroy targets, aim, fire, and control those weapons, and assess the damage they do. It would also continually report on system status to human commanders, transmit information among computer battle managers within a battle phase, and from the battle managers in one phase (e.g., boost) to the battle managers in another phase (e.g., terminal). Additional functions, unique to each phase, are described in the following sections. Each of these functions would involve making many decisions in short spaces of time using data obtained from a variety of sources. For example, aiming a weapon at a target might be based on tracking data obtained from a BSTS combined with data from a laser range finder located on satellite battle station, and would require the prediction of an intercept point for the target and weapon. Boost Phase The task of detecting booster launches would be unique to the boost phase as would be predicting the approximate trajectories of PBVs from those boosters penetrating the boostphase defense. Trajectory prediction would needed so that space-based interceptors (SBIs)

187 181 Table 7-1. Ballistic Missile Defense Battle Management Functions The components of any ballistic missile defense (BMD) system would have to be tied together in a battle management network. The table below lists the kinds of functions such a battle management system would have to perform, assuming a second- phase architecture of the type shown in Table 3-5. Computers would have to perform most of the functions. The BMD system architecture would specify locations and interrelationships among the computers. The system might be more or less centralized, more or less hierarchical. The elements of the system need to be tied together in a communications network. Chapter 8 of this report further discusses battle management communications and computation requirements. Because different system components often perform the same functions in different ways, the table gives hypothetical examples of how the functions are accomplished in different battle phases. The hypothetic/ examples are just that: this table does not purport to outline a complete BMD architecture. The table is organized into 6 sections. The first 5 sections correspond to the boost, post-boost, mid-course, terminal, and self-defense parts of a BMD battle. The sixth outlines a battle of BMD system self-defense against anti-satellite weapons. The functions and their descriptions are the same for each section of the table; what varies is the way the functions are accomplished. Thus, to find out how objects might be tracked as part of the acquisition and discrimination function during the terminal phase, one reads the hypothetical example in the section of the table devoted to the terminal phase. Boost Phase Function Description Hypothetical Boost Phase Example Acquire and discriminate objects Sense objects of interest Short and mid-wavelength infrared telescopes on BSTS detect hot exhaust plumes from launch of boosters Assess Situation Decide Course of Action Select Targets & Direct Weapons Employ Weapons Distinguish between targets to attack and decoys or debris Track objects Associate and Correlate objects sensed by different means or from different platforms Estimate whether enemy is attacking, and if so with how many of what kinds of weapons with what battle tactics Assess which of own BMD forces are available for battle Authorize firing when ready, based on direction from higher authority if available, or as preauthorized if not Determine strategy and battle plan Choose which targets to strike Assign weapons Prepare engagement instructions Assess kill: decide which targets have been destroyed Control weapon Enable weapon Prepare weapon BSTS starts track files to distinguish moving ICBMs from stationary background and cloud clutter SSTS sensors start to observe and record paths of identified objects Battle management computers compare information gathered on two separate SSTS platforms and give same identification number to the same observed objects BSTS detects ICBM launches, notes numbers and locations of launch sites, and determines types of missiles Battle management computers determine which space based space-based interceptor (SBI) carrier vehicles (CVs) are in range of launched ICBM boosters, and will be in range of Post- Boost Vehicles (PBVs) when they are released from ICBMs. If 3 or more ICBMs are launched within 1 minute, space battle management computers are programmed to command launch of spacebased SBIs when the 4th ICBM is detected Determine plan for which kinds of ICBMs from which locations and which PBVs to attack first based on trajectories. CV positions. predicted RV impact points, and predicted times of PBV separation from missile Select the booster or PBV whose trajectory will place it closest to the fly-out range of a particular CV Battle management computer decides that SBIs no and 7930 should attack target booster no. 754, and commands CVs to flightcheck SBIs. Battle management computers send flight plans and target track information to CVs. Remove a booster from the active target list Feed target information to SBI guidance package Conduct flight check of SBI

188 182 Table 7-1. Ballistic Missile Defense Battle Management Functions continued Launch weapon Fly-out and kill Open launch tube, eject and orient SBI, ignite SBI rocket motor Battle manager transmits guidance update to SBI based on SSTS tracking data; SBI homes in on target booster or PBV Post-Boost Phase Function Description Hypothetical Post Boost Phase Example Acquire and discriminate objects Sense objects of interest Infrared telescope on SSTS detects PBV after it separates from missile, and RVs and decoys after separation from PBV Assess Situation Decide Course of Action Select Targets & Direct Weapons Employ Weapons Distinguish between targets to attack and decoys or debris Track objects Associate and correlate objects sensed by different means or from different platforms Estimate whether enemy is attacking, and if so with how many of what kinds of weapons with what battle tactics Assess which of own BMD forces are available for battle Authorize firing when ready Determine Strategy and Battle Plan Choose which targets to strike Assign weapons Prepare engagement instructions Assess kill: decide which targets have been destroyed Control weapon Enable weapon Prepare weapon Launch weapon Fly-out and kill Mid-Course Phase From differences in IR signatures and other data, such as PBV recoils, sensor systems on SSTS distinguishes among PBVs, expended boosters, RVs, and decoys SSTS sensors continue to observe and record paths of identified objects Computers on two separate SSTS platforms compare information gathered by each and give same identification number to the same observed objects Battle management computers determine whichs CVs are in range of targetable PBVs and RVs Battle management computers determine plan for attacking targetable PBVs that have survived earlier SBI intercepts and when to start attacking RVs that have been deployed from PBVs Battle management computers target the PBVs that will first be in range of SBIs Battle management computer decides that space-based SBI no. 12,543 should attack PBV no. 328 and commands CVs to flight check SBIs Battle management computers send flight plans and target track information to CVs Remove a PBV from the active target list Feed target information to SBI guidance package Conduct flight check of SBI Open launch tube, eject and orient SBI, and fire SBI rocket motor Battle manager transmits guidance update to SBI; SBI homes in on target PBV or RV Function Description Hypothetical Mid-course Example Acquire and discriminate objects Sense objects of interest Infrared telescope on SSTS detects RVs and decoys Distinguish between targets to attack and decoys or debris Track objects From differences in motion after passage through dust cloud, laser range-finding radar on SSTS identifies target RVs v. decoys SSTS sensors continue to observe and record paths of identified objects

189 183 Table 7-1. Ballistic Missile Defense Battle Management Functions continued Assess Situation Decide Course of Action Select Targets & Direct Weapons Employ Weapons Associate and correlate objects sensed by different means or from different platforms Estimate whether enemy is attacking, and if so with how many of what kinds of weapons with what battle tactics Assess which of own BMD forces are available for battle Authorize firing when ready Determine Strategy and Battle Plan Choose which targets to strike Assign weapons Prepare engagement instructions Assess kill: Decide which targets have been destroyed Control weapon Enable weapon Prepare weapon Launch weapon Fly-out and kill Computers on two separate SSTS platforms compare information gathered by each and give same identification number to the same observed objects Computers determine which ERIS interceptors are in range of RVs Determine plan for which RVs to attack first Select the RVs within shortest flight time of a particular ERIS site Battle management computer decides that ERIS interceptor no should attack target RV no. 10,005 and commands fire control computer to flight check the interceptor Battle management computer sends flight plan and target track information to ERIS fire control computer Remove an RV in mid-course from the active target list Feed target information to ERIS guidance package Turn on ERIS warhead sensor Cool down ERIS homing sensor Fire ERIS rocket motor Battle manager transmits guidance updates to ERIS; ERIS homes in on target RV Terminal Phase Function Description Hypothetical Terminal Phase Example Acquire and discriminate objects Sense objects of interest Infrared telescope on AOS detects RVs and decoys based on data received from SSTS; AOS passes data to TIR Assess Situation Decide Course of Action Select Targets & Direct Weapons Distinguish between targets to attack and decoys or debris Track objects Associate and correlate objects sensed by different means or from different platforms Estimate whether enemy is attacking, and if so with how many of what kinds of weapons with what battle tactics Assess which of own BMD forces are available for battle Authorize firing when ready Determine Strategy and Battle Plan Choose which targets to strike Assign weapons Prepare engagement instructions Assess kill: Decide which targets have been destroyed From differences in motion after passage through the upper atmosphere, ground-based radar identifies target RVs v. decoys Ground-based radars continue to observe and record paths of identified objects Ground-based battle management computer compares track information handed-off by space-based battle management computer with ground-based radar data and gives same identification number to the same observed objects Computers determine which HEDIs are in range of incoming RVs Choose plan for which RVs to attack first Select the RVs nearest to a target and that can be reached by a HEDI Decide that HEDI no should attack target RV no. 257 Ready flight plan and target tracking information for HEDI Remove an RV in terminal from the active target list

190 184 Employ Weapons Control weapon Feed target information to HEDI guidance package Enable weapon Prepare weapon Launch weapon Fly-out and kill Turn on HEDI warhead sensor Cool down HEDI homing sensor Fire HEDI rocket motor Battle manager transmits guidance update to HEDI; HEDI homes in on target RV BMD System Self-Defense Function Description Hypothetical SeIf-Defense Example Acquire and discriminate objects Assess Situation Decide Course of Action Select Targets & Direct Weapons Employ Weapons Sense objects of interest Distinguish between targets to attack and decoys or debris Track objects Associate and correlate objects sensed by different means or from different platforms Estimate whether enemy is attacking, and if so with how many of what kinds of weapons with what battle tactics Assess which of own BMD forces are available for battle Authorize firing when ready Determine Strategy and Battle Plan Choose which targets to strike Assign weapons Prepare engagement instructions Assess kill: decide destroyed Control weapon Enable weapon Prepare weapon Launch weapon which targets have been Infrared telescope on SSTS detects direct - ascent ASAT SSTS sensors continue to observe and record paths of identified objects Computers on two separate SSTS platforms compare information gathered by each and give same identification number to the same observed objects Computers determine target of ASAT and which CVs may be used to defend against approaching ASAT Choose plan for which ASATs to attack first Select the ASAT nearest to a particular CV Battle management computer decides that SBI no should attack target ASAT no. 128, and commands CVs to flight check SBIs Battle management computers send flight plans and target-track information to CVs - Remove an ASAT from the active target list Feed target information to SBI guidance package Conduct flight check of SBI Open launch tube, eject and orient SBI, and fire SBI rocket motor Fly-out and kill Battle manager transmits guidance update to SBI; SBI homes in on target ASAT NOTE: The first two columns of this table draw heavily from work of Albert W. Small and P. Kathleen Groveston, Strategic Defense Battle Operations Framework, (Bedford, MA: The MITRE Corp., July 1985). The hypothetical examples are supplied by OTA. could be launched in time to intercept the PBVs before they dispensed their payloads. Post-Boost Phase Tasks unique to the post-boost phase would be noting the separation of PBV from missile and observing the PBV as it dispensed its payload. To have a chance to destroy most PBVs, the interceptors would have to have been launched during boost phase, perhaps before PBV separation. To intercept the PBVs, the system would have to guide the SBIs launched earlier. For PBVs that survived to dispense their payloads, the system might start discriminating between RVs and decoys, perhaps by trying to observe differences in PBV recoil during the dispensing process. Mid-Course Phase The primary problem for the defensive system during mid-course would be to discriminate real warheads from decoys. The number

191 185 of decoys maybe in the hundreds of thousands, or even greater. Decoys and warheads may appear very similar to optical, infrared, radar, and other sensors, both passive and active. 3 Terminal Phase During terminal phase, the defensive system would have to discriminate RVs from decoys using data handed off from earlier phases and using the atmosphere and radar signatures as discriminators. Table 7-1 shows the different functions a BMD system would have to perform during battle, and how different components would participate in different phases of the battle. The table assumes a second-phase architecture, such as described in table 3-6. It also shows the functions that would serve to defend the system against defense suppression threats. Interactions Among the Phases A BMD system would not be a single, monolithic entity. Instead, it would comprise many different elements, some of which would participate in only one or two phases of the battle. In most system architectures, battle management would be conducted by different battle managers during the different phases. Furthermore, some battle managers might be fighting one phase of the battle while others are fighting a different phase. Boost, postboost, and mid-course managers would be located in space, while terminal phase managers would likely be on the ground. For the system to function most effectively, information, such as tracks and status of RVs and decoys, would have to be communicated from battle managers in earlier phases to battle managers in later phases. Interactions for Tracking Purposes Establishing, distributing, and correlating track information is a good example of a problem involving interaction among different system elements and cooperation among battle Chapters 4 and 10 discuss the issues of discrimination during mid-course in more detail. managers. Detecting, identifying, and noting the current position of a target would not be sufficient for guiding a BMD weapon system. The target would move between the time that the sensor records its position and the time the weapon is fired. An SBI traveling at, say, 8 km/see, would take 250 seconds to reach an RV target if the SBI were fired at a range of 2,000 km from the impact point. During those 250 seconds, the RV would move 1750 km. Just as the hunter must lead the duck in flight as he fires his shotgun, the BMD system would have to aim its SBI well ahead of the speeding RV. 4 The BMD system would therefore have to keep track of each target s motion and predict where the target would be at later times. The sensors would have to generate a track file, i.e., a history of each target s motion through space. Given the target s past history in terms of position, velocity, and acceleration in three dimensions, a computer could then predict its future position. This prediction could then be used to aim and fire the weapon system. After the SBI was fired, the sensors would have to continue to track the target (and possibly the SBI), the track files would have to be updated, and mid-course guidance corrections sent to keep the SBI on a collision course. Mid-course corrections would be mandatory if the target acceleration changed after the SBI was fired, as would occur with multi-stage missiles. For directed-energy weapons or interactive sensor systems, the delay from the time that energy is emitted by the target until it reaches the sensor, and then from the firing of the weapon until the arrival of the kill energy traveling at the speed of light, would be very short, but not zero. At 2,000 km range, for example, 13 milliseconds would elapse from the last sensor reading until the time a laser beam could reach the target. The RV would move about 91 meters in this time, so some predictive ca- 4 However, the RV would be moving on a ballistic or free-fall flight with no external acceleration other than the force of gravity. Therefore, predicting its future path or trajectory would be possible provided that the sensor generated two or more accurate three dimensional target positions. Predicting the future path of an accelerating, multi-stage missile would be much more difficult. RVs that could maneuver would worsen the difficulties

192 186 pability would be required. In addition, the observable characteristics of the target would change drastically during the tracking operation. The sensor systems would begin tracking the hot booster plume. For boost phase kills, the sensors would have to acquire and track the cold booster body, or rely on calculations of the missile body position relative to the booster plume for all booster/sensor orientations. After the last booster stage had burned out, the battle management computers would have to continue the track file on the surviving post-boost vehicles (PBV), even though tracking data might be derived from other types of sensors. Finally, as individual RV s and decoys were deployed, the track files would have to proliferate, taking the last PBV track projection as the recently deployed RV track file, until it could be verified and updated by long wavelength infrared sensors. The data handling problem would be compounded by RV s that survived the boost and post-boost defensive attacks. The surviving RV s would usually pass out of the field of view of the initial sensor. The track file obtained from one sensor s data would then have to be correlated with the data from another sensor. 6 Track data would be passed to the appropriate weapons platform at each stage of the battle. Eventually the track files of surviving targets should be passed on to the ground-based terminal defensive systems to aid in the final kill attempts. Information on decoys and other rocket or killed target debris should also be In general it would take additional time for the sensor signal processor to analyze the sensor data and for the track file to be updated after the last signal was received; the actual elapsed time between observation and the order to fire the weapon might be 5 to 10 seconds, so the target might move as much as 70 km even for a directed-energy weapon. 6 If battle managers, sensors, and weapons were organized into autonomous battle groups, then each battle manager would have to hand off and receive track data as targets passed through the field of view of sensors in its group, or it would have to perform all of its own target acquisition and discrimination. If there were a single battle manager to handle all tracking and correlation, it would have to maintain track files on all targets. Such an organization would tie system survivability to survivability of the central battle manager. Finally, if battle groups were to use fixed battle management platforms, but different sensors and weapons as the battle continued, then the battle managers would have to correlate tracks from different sensors as the sensors moved in and out of its group. transmitted, to avoid attacking too many false targets. System Performance and Interaction The ability to correlate data well from different sensors (required to get accurate threedimensional track histories) could have a strong effect on system performance, as could the ability to correlate track files exchanged among battle managers both within and between phases. Poor performance in the early phases would mean many RVs leaking into later phases, with possible overload of resources assigned to mid-course and terminal phases. Distinctions Among the Phases In all phases the defensive system would have to perform many similar functions, such as tracking; weapons assignment; aiming, firing and controlling weapons; and reporting status. Sensors and weapons would vary considerably from one phase to another, however. For example, boost, post-boost, and mid-course tracking would be done primarily by spacebased infrared sensors such as those incorporated in the BSTS and SSTS systems. Terminal phase tracking would be done by a combination of airborne infrared sensors and ground-based radars. The software and hardware used to perform the sensing, discrimination, and tracking functions indifferent phases would likely be quite different; aiming, firing, and controlling weapons might be similar. Some phases would require unique functions. A good example is interactive discrimination of RVs from decoys for mid-course defense. Candidates for such discrimination, such as neutral particle beam systems, would likely be controlled by unique computer software and hardware adapted to the task. Reconfiguration In addition to fighting the battle, the system would also have to be able to reconfigure itself during and after the battle, to compensate for damage done to it, in preparation for further or continued engagements. In the post-

193 battle case, this might be done with human assistance. Opportunities for Human Intervention Tracking and discriminating objects, aiming and firing weapons, and managing the battle would require computers. During peacetime, humans could monitor surveillance data after it had been processed and displayed in a form suitable for human interpretation. Human decisionmakers could deduce from the events monitored, among other things, whether and when the defensive system should be placed in alert status, ready to cope with a battle. Once the battle started, however, the response time and data processing requirements would severely limit the opportunities for human intervention. There are four possible human intervention points under consideration in currently suggested BMD architectures: the decision to move the system from peacetime status to alert status, the decision to release weapons, the decision to switch to a back-up for one or more of the algorithms (see box 7-A) used by the battle management computers, and selection from a pre-specified set of tactics for terminal phase, made as a result of observations of earlier phases of the battle. Transition to Alert Status Humans could decide to move from one level of alert to another in hours or minutes, as compared to fractions of seconds for computers

194 188 Box 7-A. Algorithms Methods for solving problems by the use of computers are often expressed as algorithms. As described by John Shore, An algorithm is a precise description of a method for solving a particular problem using operations or actions from a well-understood repertoire. 1 More technical definitions often require that the description contain a finite number of steps, each of which can be performed in a finite amount of time, and that there be specific inputs and outputs. As explained by computer scientist Donald Knuth, The modern meaning of algorithm is quite similar to that of recipe, process, method, technique, procedure, routine,...2 Carrying out the steps of an algorithm is known as executing it. If one thinks of a recipe for baking a cake as an algorithm, then executing the algorithm consists of following the recipe to produce the cake. The following is a simplified example of an algorithm that might be used in the early stages of the design of a BMD system. The purpose of the algorithm is to detect the launch of boosters. We assume that the system uses a sensor on a satellite that can scan the Soviet Union. The sensor is composed of a number of different elements, each of which is sensitive to the radiation emitted by a booster. The Soviet Union is divided into regions, and each detector element periodically scans sequentially across a number of regions For each detector element, record all detected radiation sources greater than the threshold for a booster as the detector scans across the Soviet Union. Record the time of occurrence of each detection as well as the intensity of the source. For each source recorded, identify its region of origin. Compare the occurrences of sources in the current scan with occurrences from the previous two scans. Count all events consisting of occurrences of sources in the same region for three consecutive scans. Flag each such event as a launch. If data have been saved from more than 2 consecutive scans, discard the data from the oldest scan and save the data from the current scan. If no launches were observed, go back to step 1, otherwise continue with step 4. If launches were observed, notify the system operator While this description is simplified, e.g., omitting consideration of booster movement across regions, it is an algorithm because the operations needed to perform each step could be completely specified; furthermore, it could be implemented as a computer program. Although the number of steps used in describing an algorithm must be finite, the definition does not require that the algorithm terminate when executed. Many algorithms are designed to be non-terminating, such as the following simplified description of how a radar processing system might operate: 1. Send out radar pulse. 2. Wait a pre-calculated interval for a return pulse. 3. If there was a return pulse calculate the distance to the object. 4. Go back to step 1. Despite not terminating, this algorithm still produces useful results. Some algorithms terminate under certain conditions, but do not terminate and produce no results under other conditions. Conditions under which algorithms do not produce the desired results are known as error or exception conditions and the occurrence of such conditions as undesired events. For the following simplified algorithm, which tracks a target based on radar returns, the failure of the radar pointing mechanism is an undesired event that causes the algorithm to continue endlessly, producing no useful result. 1. Retrieve the last known target location, velocity, and acceleration. 2. Calculate the estimated current target location. John Shore, The Sachertorte Algorithm and Other Antidotes to Computer Ati ety (New York, NY: Viking Press, 1985), p Donald E. Knuth, The Art of Computer Programrm ng, Vol. 1: Fundamental Algorithms (Reading, MA: Addison Wesley, 1974), p. 4.

195 189 I A Point the radar at the estimated current target location and attempt object detection. If the radar locates no object at the estimated range to the target, revise the estimated target location and return to step 3. Otherwise, continue with step 5. If the radar locates an object at the estimated range to the target, update the target location, velocity, and acceleration. real version of such a tracking algorithm would have to take into account the possibility that the radar might fail in any of several different ways, or that earlier estimates of target position might be grossly wrong. during battle. The humans decisions could be based on data gathered from sources both within and outside the BMD system. The computerized battle-time decisions would be based on data acquired by the system s sensors. Weapons Release Once a human had permitted the transition to the highest level of alert, the system would function automatically, responding to threats as it perceived them. 7 It would be possible to build human intervention points for the release of weapons into the battle management process. In the first-phase and second-phase systems described in chapter 3, the first weapons to be released would be the SBIs. The period from the time that a missile launch was first perceived by BMD sensors until SBIs would have to be launched to intercept a missile still in its boost phase would be quite short a few minutes at most. Accordingly, if humans were to control the release of weapons, they would have to monitor the defense system s operation continuously once it had moved to the highest alert status. Since it may be necessary to release hundreds or thousands of SBIs within minutes, a human operator would not be able to authorize release of individual SBIs. Because of the rapid reaction times needed, continual human intervention in the weapons release process would likely degrade system effectiveness unacceptably. It might be feasible to intervene when previously unused weapon systems were The AEGIS ship defense system, often cited as performing many of the same functions as a BMD system, reacts completely automatically to incoming threats when in the highest level of alert mode. For some threats, AEGIS must react within 15 seconds from the time a threat is detected. brought into the battle. As an example, if neutral particle beam (NPB) weapons had not been used before enemy missiles reached midcourse, then a human might be called onto authorize their use during the mid-course part of the battle. Even such occasional intervention might degrade performance somewhat. Switching to Back-ups During the course of the battle it might be possible for a human observer to determine that a BMD system was malfunctioning. For a human to notice, the malfunction would probably have to be gross, such as a failure to fire interceptors or firing interceptors in obviously wrong directions. If the problem lay in the algorithms used by the battle management computers, and if the system were designed in such away that back-up algorithms were available to the computers, then the human might command the battle management computers to switch to a particular back-up algorithm. 8 Human intervention of this type is rarely used in existing systems because the human cannot interpret the situation correctly in the available time, and because it is difficult to design the software to switch algorithms successfully in mid-computation. Inmost systems, the gain is not worth the added software complexity. The potential gain for BMD from such intervention would be that in the cases where Switching to a back-up algorithm should not be confused with situations where a computer uses built-in diagnostics to determine the occurrence of a hardware malfunction and then automatically switches operation to a redundant component. Such diagnostics and hardware redundancy for automatic switching are now used in some critical applications, such as airline transaction systems, telephone switching systems, and battle management systems such as AEGIS.

196 190 the system had been badly spoofed by the enemy, and the human operator quickly recognized the symptoms, cause, and needed corrective action, recovery might be possible in time to continue the battle. The risk would be that the operator would misjudge the situation, or that the complications involved in provialing the appropriate interface to the operator, both in additional software, hardware, and communications capability, would make the system less reliable. Selection of Tactics Because the boost, post-boost, and midcourse phases of a BMD battle would last minutes, a human commander might be able to evaluate the results of those phases in time to affect the tactics used during the terminal phase. To do so, he would have to be presented with status reports during the battle. Based on his analysis of the battle, and on choices of previously-determined tactics presented to him by an automated battle manager, he could choose the terminal phase tactics to be used. Again, because of the time-scales and data volumes involved, he would probably not be able to alter his choice once the terminal phase began. Increasing degrees of human intervention would require increasing complexity in the interface between humans and the battle management system. A sophisticated interface between human and computer would be needed, allowing the human to observe status and issue commands, and, when appropriate, receive acknowledgements. Such an interface would add complexity to the software. Furthermore, the human operator(s) would probably have to have authority to release weapons, as there might not be time to consult with higher authorities. Common to all BMD system designs that require human intervention at any stage is the need to provide secure, rapid communications between the human and the battle management computers. If part of the system were in space, then most likely there would be a need for space-to-ground communications. 9 For all of the preceding reasons, it seems likely that a BMD system would operate almost completely automatically once moved to an alert status in preparation for battle. The preceding analysis illustrates the difficult trade-offs involved in designing a battle management system. In considering the interface between humans and the system, the designer must trade off communications security against the need for human intervention against system structure against complication of the computing tasks. He must also balance system performance against all other considerations, deciding whether the system could perform better and more dependably with the aid of a human than without, and whether any extra complication in the human-computer interfaces would be worth whatever capability and trustworthiness might be added by the human. Even if a human operator were space-based, he might need to communicate with higher authority on the ground. Such communications would probably not require as rapid data communication rates as battle-management-to-operator communications. SUPPORTING Table 7-1 shows the primary battle management functions, but does not include several supporting functions. Most important of these are communications and recovery from damage and from failures. Both of these functions are needed in all phases, with communications playing its traditionally crucial role in battle management and with recovery invoked as FUNCTIONS needed. Both communications and recovery procedures would be completely automated during a battle. Because of the short decision times involved during boost, post-boost, and terminal phases, recovery would have to be extremely rapid; delays would result in RVs moving on to the next phase or reaching their targets. Long delays in recovery could also reduce

197 191 the opportunities for a battle manager fighting in one phase to pass information along to battle managers fighting in the next phase. Communications Automated communications links between sensors and battle management computers, between different battle management computers, between battle management computers and weapons, and between battle management computers and humans, would all be needed for effective battle management. Data would be continually transmitted over a battle management communications network during all battle phases. Recovery From Damage and Failures Present in all phases would be the need to recognize and recover from system failures and from damage to system resources. Individual system components would have to monitor and report their own status continually. They would have to try to recover from local failures, whether internally generated perhaps by a software error-or externally generated perhaps by a detonation of a nearby nuclear antisatellite weapon causing radiation damage in a computer chip. Some instances of system damage and failure, such as destruction of several adjacent battle management platforms or communications controllers, would require recovery based on global information, i.e., information about the status of the entire BMD system and the entire battle. Examples are knowing how to reroute communications around damaged nodes in the communications network, or knowing which battle management computers were in position, both physically and in terms of resources available, to take over the functions of a disabled battle manager. COMPLEXITY OF BATTLE MANAGEMENT Conduct of a successful BMD battle would be similar to the conduct of a large conventional battle in that it would require the orchestration of many different kinds of components under precise timing constraints. The problem may be ameliorated somewhat by preplanning some of the orchestration. The difference is that in a BMD battle the time constraints would be tighter, the battle space would be larger, the fighting would largely be automated, the components would be previously untested in battle, and there would be little chance to employ human ingenuity to counter unanticipated threats or strategies. The only kind of BMD system for which the U.S. has battle management software development experience and an understanding of the attendant problems is a terminal defense system, such as SAFEGUARD. Some consider even this experience as suspect, since SAFEGUARD and other systems like it were never used in a real battle. Adding a boostphase defense would add complexity to the system and require the inclusion of technologies hitherto untried in battle. It would also be the first time that software was used to control highly automated space-based weapons. Adding amid-course defense would probably increase the software complexity past that of any existing systems. The burden of effectively integrating information from different sensors, controlling different weapons, coordinating interactive discrimination to distinguish among hundreds of thousands of potential targets, and selecting effective strategy and tactics all while trying to defend against active countermeasures-would fall on the software. (Software issues are discussed in detail in ch. 9.) Approaches to reducing complexity center on divide and conquer strategies applied to architecture definition, aided by simulations of the effectiveness of different battle management architectures. Those who favor such approaches believe that the system could be designed and built in small, relatively inde-

198 192 pendent pieces that could each be adequately tested, and that could be jointly subjected to peacetime tests. As an example, each interceptor carrier vehicle might contain a battle manager, designed to fight independently of other battle managers. They argue that the system could be easily expanded by adding more pieces, e.g., more CVs each with its own battle manager. Since the pieces would tend to be independent, the reliability of the system would be more strongly related to the reliability of an individual piece, rather than to the joint reliability of all pieces, i.e., knowing that individual pieces were reliable would suggest that the whole system was reliable. Also, the failure of a single piece might not be as catastrophic as in an architecture where the pieces were highly interdependent. Those who doubt the effectiveness of such a strategy in the face of the complexity induced by BMD requirements argue that making the pieces independent would require making them very complex. They further note that historically no approach has led to the development of a weapon system whose software worked correctly the first time it was used in battle. The greater complexity of BMD software over existing weapon systems leads them to believe that a BMD system would have little chance of doing so. BATTLE MANAGEMENT ARCHITECTURE A battle management architecture is a specification of the battle management functions to be performed by different system components and the relationships among those components. Components may be software, such as a set of computer programs that would allocate weapons to targets, or hardware, such as the computer(s) used to execute those programs. (See also ch. 3.) A significant architectural tradeoff concerns the degree of coupling among battle managers (see box 7-B). Some proposed architectures use a very loosely coupled system with little communication among battle managers, similar to the almost perfect architecture described in the Fletcher Report. 10 Such architectures tend to locate battle managers on board carrier vehicles. Others use a more tightly coupled system with track and other data exchanged among battle managers for coordination purposes. They often locate battle managers on separate platforms. IOJ~es C. Fle~her, Study Chairman tmd B. IMcMill~, p~el Chairman, Report of the Study on Mimi.mitingtbe Threat Posed by Nuclear Balh stic Missiles: Volume V, Battle Management, Commum cations, and Data Processing, (Washington, DC: De partment of Defense, Defensive Technologies Study Team, Oct. 1983), p. 19. Box 7-B. Centralization, Distribution, and Coupling A centralized system concentrates computing resources in one location and may consist of several processors that share the same memory and are housed together physically. Such a system is known as a multiprocessor. The processors are able to communicate with each other at very high data rates, and are said to be ightly coupled. As the processors are physically moved apart, acquire their own, separate memories, and as the data communication rates among them decrease, they acquire the characteristics of a distributed system, also called decentralized, and are said to be loosely coupled. An important factor in understanding the degree of coupling is the criterion used to partition the battle space into segments so that each battle management computer has responsibility for a segment. Indeed, criteria for segmentation are one way of distinguishing among architectures. Segments might be geographically determined and of fixed location, or might be determined by the clustering of targets as they move through space, or might be determined by the location of battle

199 193 managers, CVs sensors, and other system resources during a battle. on local data. As yet, few detailed studies of such technical issues appear to have been made. The Eastport Group recommended the development of a decentralized, hierarchical battle management architecture. 3 Architectures currently under consideration for BMD systems are consistent with that recommendation. In a typical such architecture, each battle manager would report as necessary to battle managers at higher levels, and would receive commands from them. There might be 3 layers of battle managers; the lowest layer would be local battle managers, which perform the fighting functions. The next layer would be regional. 13The EastPort Study Group is the name used to refer to the SDIO Panel on Computing in Support of Battle Management. It was appointed to devise an appropriate computational/communication response to the SDI battle management computing problem and make recommendations for a research and technology development program to implement the response. Its Although the Eastport Group recommended that BMD battle managers be hierarchically structured, the Fletcher Report 12 suggested that a logical battle management structure that was almost perfect would not be hierarchical, but would consist of a single battle manager replicated a number of different times, with each copy physically located on a different platform. The Fletcher report also noted that such an architecture might be very costly, that there might be equally effective and cheaper alternatives, and that it was important to look at technical issues that distinguished among those alternatives. An example given was the effectiveness of algorithms that allocated weapons to targets based only lEastport Study Group Report, Report to the Director, Strategic Defense Initiative Organization, z Fletcher Report, op. cit~ footnote 10, pp report was issued in-de;ember Box 7-C. Hierarchies and System Design Designers of systems find it useful to impose a structure on the design. For complex systems, several different structures may be used, each allowing the designer to concentrate on a different concern. In systems where many components are involved, such as complex software systems, large communications systems, and complex weapons systems, the structures used are often hierarchical. Each hierarchy may be specified by identifying the participating components and a relationship among them. The military command structure is an example of a hierarchy. The components are individuals of different rank, and the relationship is obeys the commands of, e.g., a lieutenant obeys the commands of a captain. Many proposed SDI battle management architectures use some variation of the relationship resource contentions are resolved by. Thus, local battle managers resource contentions are resolved by regional battle managers. However, another important battle management hierarchy is communicates track data to. This latter hierarchy is important in determining communications needs for the BMD system, and is sometimes confused with the former. A tree is a special form of hierarchy in which a component at one level is only related to one component at the next higher level. The military chain of command is an example. A lieutenant is only commanded by one captain, although a captain may have several lieutenants under his command. The Eastport Group considered battle management architectures that were structured as trees to be the most promising for SDI. Structures may describe relationships among entities in a design, independent of physical relationships among system components. Such structures are sometimes known as logical structures. 2 The Eastport Study Group is the name used to refer to the SDIO Panel on Computing in Support of Battle Management. It was appointed to devise an appropriate computationallcommunication response to the SDI battle management computing problem and make recommendations for a research and technology development program to implement the response. Its report was issued in December 1985.!%, for example, Report of the Study on Eliminating the Threat Posed by Nucfear BaJlistic Missdes, James C. Fletcher, Study Chairman, Volume V, Battle Management, Communications, and Data Processing, (Washington, DC: Department of Defense, Defensive Technologies Study Team, October 1983), p. 18.

200 194 battle managers, which would resolve contentions for resources among local battle managers, as battle managers, sensors, and weapons moved, and which would assign responsibility for targets passing between battle spaces. At the top would be a global battle manager that would establish strategy for the regional and local battle managers and that which would provide the interface between humans and the system. The battle space would be partitioned into segments such that each battle manager in the lowest layer of the hierarchy had responsibility for a segment. As battle managers and targets moved through the battle space, information concerning them, such as type of target, location of target, and trajectory of target, would have to be moved from one computer to another. Some recent proposals have suggested fewer layers in the battle management hierarchy. In such architectures, the hierarchy of automated battle managers is flat, i.e., there are no regional battle managers, and the top layer is a human commander. Such organizations have been designed so that battle managers may act almost independently of each other. The volume of data to be communicated among the battle management computers would depend on the degree of coordination among the battle managers required by the battle management architecture. (See chapter 8 for estimates of communication requirements.) The determining factor is the amount of target tracking information that would have to be exchanged. Since there might be hundreds of thousands of objects to be tracked during mid-course, architectures that required tracks of all objects to be exchanged among battle managers would place a heavier load on communications than those that required no object tracks to be exchanged. The price paid for exchanging less information, however, would be the traditional one: the ability to coordinate the actions of different battle managers would be hampered and the overall efficiency of battle management might be decreased. The efficiency-volume trade-off may be seen by considering the transition from one phase of the battle to the next. As an example, the terminal-phase battle managers would have the best chance to destroy targets if they received target-tracking information from the mid-course battle managers. Without such information they would have to acquire, track, and discriminate among targets before pointing and firing weapons. With such information they would only have to continue tracking targets and point and fire weapons. In such a situation, one might suggest combining the midcourse and terminal-phase battle managers into one set of programs on one computer as opposed to a more distributed system withinformation transmitted among battle managers. Unfortunately, this organization would probably complicate the battle management task, since there would be somewhat different functions to be performed in the different phases, and since the way functions would be implemented in different phases would be different. 14 The Eastport Group believed that a hierarchical battle management organization would simplify the computing job of each battle manager and would allow the battle managers to act without frequent interchange of information. 15 The concerns of each battle manager could then be simplified more than in a nonhierarchical organization, battle managers could be added to the system as needed, and the system would still survive if a few lower level battle managers were lost. 14 Since different weapons would be used in the terminal phase as compared to the mid-course, pointing and controlling the weapons would be done differently. Similarly, different sensors may be used to discriminate between targets and decoys, requiring the allocation of resources with different characteristics and therefore a different resource allocation algorithm. Some earlier proposed architectures required the battle managers to be tightly coupled, exchanging considerable information with each other frequently. The Eastport Study Group rejected such an architecture because of the computing and communications burden it would place on the battle managers, and because of the complexity it would induce in the battle management software.

201 195 Decentralizing battle management means that the battle management task would be physically distributed among different computers. Decentralization would permit other battle managers to continue fighting even if a local battle manager were disabled. However, if the degree of coupling were high, the loss of data from the disabled battle manager might result in reduced effectiveness of the others. Without a specific design and a way of effectively testing architectures, it is difficult to verify claims about their merits and deficiencies. Such tests would have to be based on simulations and on whatever peacetime tests could be conducted. 16 However, the apparent disadvantages of a decentralized, hierarchical system would be: contentions for resources would have to be resolved at upper levels of the hierarchy, possibly adding complexity to the computational problem as a whole, the actions of battle managers would be based mainly on local data, perhaps resulting in inefficiencies, e.g., adjacent battle managers might both shoot at some of the same targets, thereby wasting shots, un-. The proposed National Test Bed, might provide some of the simulation capabilities needed for architecture evaluation. less sophisticated battle management algorithms to compensate for the information loss could be developed,. if strategic and tactical decisionmaking were concentrated at one level in the hierarchy, disabling some or all of that level could greatly reduce system performance. Such damage would be easier to accomplish if there were relatively few battle managers at that level, as might be true at the higher levels of the hierarchy. The Eastport group believed that the advantages of a hierarchical, decentralized system far outweighed the disadvantages. Evaluation of advantages and disadvantages must await a design specific enough to be tested, and an effective test method. No matter the choice of structure for battle management, some technology would be strained and software dependability would be a key issue. Centralization would appear particularly to stress computational performance and survivability. Decentralization would appear to require more sophisticated software at the local battle manager level and would increase the weapons supply needed. All architectures would require secure communication, whether to exchange track data, or to receive sensor data, or to communicate with the ground. CONCLUSIONS Ballistic missile defense battle management would be an extremely complex process. The number of objects, volume of space, and speed at which decisions would have to be made during a battle preclude most human participation. Aside from authorizing the system to move to alert status, prepared to fight automatically, at best the human s role would be to authorize the initial release of weapons and to change to back-up, previously-prepared, strategy or tactics. Decisions about which weapons to use, when to use them, and against which targets to use them would all be automated. Inclusion of human intervention points would likely add complexity to an already complex system and to compromise system performance in some situations. On the other hand, if an attacker had successfully foiled the primary defensive strategy, human intervention might allow recovery from defeat. Battle management architectures as yet proposed are not specific enough for their claimed advantages and disadvantages to be effectively evaluated. Such evaluation must await both better architecture specifications and the development of an effective evaluation technique, perhaps based on simulation.

202 196 w

203 Chapter 8 Computing Technology

204 CONTENTS Introduction The Need for Automation Integrating Sensors, Weapons, and Computers Communications and Computer Networks Communications Networks Simulations and the National Test Bed....., Simulations and Systems Development Current Battle Simulation Technology , The National Test Bed Computers in Support of BMD System Development Computing Technology Trade-offs Processor Technology Capabilities of Existing Computers New Computer Architectures Space Qualification and Radiation Hardening Conclusions ,,.215 Reliable, Secure Communications Simulations Technology and Architectural Trade-offs Computational Requirements Box Boxes 8-A. Simulations ,206 8-B. MIPS, MOPS, and MEGAFLOPS ,213 Table No. Tables 8-l. Computers in a Ballistic Missile Defense System Computing Performance Requirements Page Page

205 Chapter 8 Computing Technology INTRODUCTION This chapter discusses the demands that ballistic missile defense (BMD) systems would place on computing technology, and the tradeoffs that would have to be considered in satisfying those demands. Initial sections discuss why BMD would need computers and how it would use them for battle management, weapons control, sensor data processing, communications, and simulation. Later sections describe the technology used to build computers and the requirements that the Strategic Defense Initiative (SDI) imposes on that technology. The chapter concludes with key issues posed by SDI computing needs. Any description of computing technology must be accompanied by a discussion of software and software technology issues; these can be found in chapter 9 and appendix A. The Need for Automation The rapid response times and volume of data to be processed would require the use of computers in every major BMD component and during every phase of battle. Humans could not make decisions fast enough to direct the battle. The launch of thousands of intercontinental ballistic missiles (ICBMs), some employing fast-burn rocket boosters, might permit less than 60 seconds to detect, track, aim, and fire weapons at the first boosters to clear the atmosphere. During mid-course it might be necessary to account for a million objects and to discriminate among hundreds of thousands of decoys and thousands of reentry vehicles (RVs). In the terminal stage, RVs in the atmosphere would have to be quickly located, tracked, and destroyed by interceptor missiles. Mutual occupation of space by two defensive systems of comparable capabilities might require considerably faster reaction times than Note: Complete definitions of acronyms and intialisms are listed in Appendix B of this report. those needed to meet a ballistic missile attack alone. Countering an attack by space-borne directed-energy weapons would require response times of seconds or less to avoid the loss of critical defensive capabilities. The critical part of such a battle could well be over before humans realized that it was taking place. Although automated decision-making is a focus of concern for the use of computers in BMD, computers would also serve many other purposes. Table 8-1 shows many of the places where computers would be used. Integrating Sensors, Weapons, and Computers An automated BMD system would require some degree of coordination among different computers, but there would be many places where computers would act independently of each other. Table 8-1 shows many such cases, e.g., computers incorporated into sensor systems, such as radars, to perform signal processing on data perceived by the sensor. In each case the computer may be specially designed for its job and is physically a part of the system of which it is a component. As an example, an imaging radar would build up an image of an object such as an RV by analyzing the returns from the object over a period of seconds. The radar would process each return individually and store the results. With sufficient individual returns, the radar could analyze them to form an image of the object. A single computer incorporated into the radar would perform the processing, storage, and analysis. From the viewpoint of an external observer, such as a battle management computer residing on a different platform, the radar is a black box that produces an image of an RV. The external observer need know nothing about the computer inside the 199

206 200 Component First. and Second. Phase Systems: Battle Management Computers a Boost Phase Surveillance and Tracking Satellite (BSTS) Space Surveillance and Tracking Satellite (SSTS) Laser Thermal Tagger Carrier Vehicle (CV) for Spacebased Interceptors (SBI) Space-based Interceptor (SBI) Airborne Optical System (AOS) Exe-atmospheric Interceptor System (ERIS) Ground-based Terminal Imaging Radar High Endo-atmospheric Interceptors (HEDI) Third Phase, Add: Ground-based Laser, Space-based Mirrors Space-based Neutral Particle Beam (NPB) Table 8-1. Computers in a Ballistic Missile Defense System Purpose of Computers Coordinate track data (e.g, maintain a track data base and correlate data from multiple sensors); maintain status of and control defense assets; select strategy; select targets; command firing of weapons; assess situation. Process signals to transform IR sensor data into digital data representing potential booster tracks; process images to recognize missile launches and to produce crudely-resolved booster tracks; communicate with battle management computers; maintain satellite platform: guidance, station keeping, defensive maneuvering; housekeeping. Process signals to transform IR, laser range-finder, and radar sensor data into digital data representing potential tracks; process images and data for fine-tracking of launched boosters, post-boost vehicles, RVs and decoys and to discriminate RVs from decoys; point sensors; communicate with other elements of the BMD system; guidance, station keeping, defensive maneuvering, housekeeping (maintain mechanical and electronic systems). Point the laser beam; communicate with other elements of the BMD system; maintain satellite platform: guidance, station keeping, defensive maneuvering; housekeeping. Monitor status of SBIs; control launching of SBIs; communicate with battle manager; maintain satellite platform: guidance, station keeping, defensive maneuvering; housekeeping. Guide flight based on commands received from battle manager; Track target and guide missile home to target; communicate with battle manager; housekeeping. Process signals to transform IR data into digital data representing potential tracks; process images and data for fine-tracking of post-boost vehicles, RVs and decoys and, if possible, discriminating RVs from decoys; point sensors; communicate with other elements of the BMD system; control of airborne platform; housekeeping. Guide flight; process signals and images from on-board sensor for terminal guidance and target tracking; communicate with battle manager and SSTS, AOS, and probe sensors; housekeeping. Process signals and images to convert radar returns to target tracks; process images and data to discriminate between decoys and RVs; control radar beam; communicate with battle managers and other elements of BMD system; housekeeping. Guide missile flight based on commands received from battle manager; process signals and images from on-board sensor for terminal guidance and target tracking; communicating with battle manager; housekeeping. Manage laser beam generation; Control corrections to beam and mirrors for atmospheric turbulence; steer mirrors; communicate with battle manager; housekeeping. Manage particle beam generation; steer the accelerator; track potential targets; communicate with battle manager and neutron detector; maintain satellite platform: guidance, station keeping, defensive maneuvering, housekeeping. Radiation Detector Satellites Discriminate between targets and decoys based on sensor inputs; communicate with battle manager and/or SSTS; maintain satellite platform: guidance, station keeping, defensive maneuvering; housekeeping. amay be carried on sensor platforms, weapon platforms, or separate platforms; ground-based units may be mobile. SOURCE: Office of Technology Assessment, 1988,

207 201 radar, or how it operates, but only the form and content of its output. Customizing the Computer for the Application The above black box design strategy is based on sound engineering principles and tends to simplify the battle management architecture, but it still involves some difficult trade-offs. One such trade-off is that between developing special-purpose computers for different sensors and weapons versus utilizing commercially available hardware. Utilizing commercially available computers may simplify the job of software development. There would be people available who have experience with existing hardware. In addition, support tools for software development on available computers already exist. As a result, software developed for commercially available computers would probably be more reliable, more efficient, and less expensive than software developed for new computers built specifically for BMD. Furthermore, software development would not have to wait for development of the hardware, reducing the risk of not meeting schedules. On the other hand, hardware specially built for BMD is likely to be more efficient and better suited to the job, possibly offsetting efficiency losses in software. Moreover, maintainability, reliability of the hardware, and life-cycle cost would have to be taken into account. Software experts at OTA s SDI Software Workshop suggested that hardware customization v. software reliability and cost was an important trade-off that should be resolved in favor of simplifying software development. However, some SDI computing might require the use of novel hardware designs, even though this might require designing new and complex software from scratch. Communications and Computer Networks Battle management requires communications among the battle managers, sensors, and weapons forming a BMD system and between the battle managers and the human operators of the system. Space-to-space, space-to-ground, and ground-to-space communications would be required. As in traditional battle management, information must be sent in useful form, on time, and securely to the place where it is needed. Also as in traditional battle management, information transmitted among battle managers concerns the location of targets and weapons, the status of resources, and decisions that have been made. Distinct from traditional battle management, information transmitted in a BMD system would all be digitally encoded and the transmissions controlled by computers. As noted in chapter 7, the rate and volume of data to be transmitted depend on the battle management architecture. Estimates of Communications Requirements The Fletcher Report estimated that the peak data rate needed by any communications channel in a BMD system would be about 10 7 bits per second (bps). This estimate assumed that an entire track file would have to be transmitted, that the file would contain 30,000 tracks, and that each track could be represented in 200 bits. Except for the number of tracks in a track file, the estimate is based on conservative assumptions. Furthermore, it scales linearly with the number of tracks, i.e., a track file containing 300,000 tracks would require a peak rate of about 10 8 bps. In more recent work, analysts have made more specific assumptions about architectures and have been able to produce more refined, but still rough, estimates. For example, one study of boost-phase communications uses a highly distributed architecture consisting of sensor satellites and satellite battle groups composed of battle management computers, sensors for booster tracking, and space-based interceptor (SBI) carrier satellites. Additional assumptions were made about numbers of targets tracked per sensor in the battle, number James C. Fletcher, Study Chairman and B. McMi&n, Panel Chairman, Report of the Study on E!inn nating the Threat Posed by Nuclear Ballistic Missiles: Volume V, BattJe Management, Communications, and Data Processing, (Washington, DC: Department of Defense, Defensive Technologies Study Team, Oct. 1983), p. 19.

208 202 of bits per target track, non-uniform message traffic density, number of relays per message, varying message types (examples are track data, status information, and engagement data), and number of seconds per frame. The result was a peak link data rate for boost phase within the transmission rates of current technology. 2 The Fletcher Report noted, and OTA concurs, that: The technology exists today to transmit 10 7 to 10 8 bits/sec over data links of the length and kind needed for a BMD system. Therefore, even with 300,000 objects in the track file, existing communication technology could handle the expected data rates. Cost and complexity will vary with the rate designed for, but the Panel concludes that communication rates, per se, will not be a limiting factor in the design of a BMD system. 3 Communications Networks Regardless of volume, communications would have to be secure and reliable. It would have to survive attempts by an enemy to intercept, jam, or spoof communications, at best rendering the system ineffective, at worst taking control of it for his own purposes. It would also have to survive physical damage incurred in a battle or defense suppression attack. Understanding the threats requires understanding how communications would function in a BMD system. Current communications technology, including that proposed for BMD systems, involves establishing a network of computers, each acting as a communications node, that transmit data to each other. One example of an existing network that is widely distributed geographically is the ARPA network, initially developed by the Defense Advanced Research Projects Agency (DARPA) as an experimental network. Another example is the AT&T long distance telephone network. Both differ considerably from a space-based battle communications network, which would have: Personal communication, Ira Richer, The Mitre Corp. The Fletcher Report, op. a-t., footnote 1, p more nodes and more available direct connections between nodes; different delays between nodes (perhaps 5 milliseconds for the example distributed space-based network described earlier as compared to more than 25 milliseconds for the ARPA network); more stringent security requirements; a need to re-establish links every few minutes; and, probably long repair times for individual nodes. Nonetheless, the problems are sufficiently similar that the terrestrial networks are useful examples. Each node in the network communicates with several other nodes. Users of the network communicate by submitting messages to the network. 4 The computers controlling and comprising the network route the message from one node to another until it reaches its destination. Some of the major issues that must be resolved in designing a communications network are: the physical arrangement of the nodes and the interconnections among the nodes; the unit of data transmission, which may be a complete message or part of a message; the algorithm used to decide what route through the network each unit of data transmission will take; the algorithm used to encode units of data transmission so that they may be reliably transmitted; the algorithm used by nodes for interchanging data so that the start and end of each data transmission may be determined; and the methods used to ensure that data communications are secure and cannot be jammed, spoofed, or otherwise rendered unreliable. 4 In the AT&T network, messages are sent across the network to establish a circuit to be used for a long distance call when a subscriber dials a long distance number. Generally, once a circuit is established, it is dedicated to a call, and communications on it may be sent in non-message form as analogue signals or may be encoded digitally into messages.

209 204 Message Transmission Information to be sent over a digital communications network, such as used in BMD systems, is organized into messages. In some networks, known as packet-switched networks, for transmission purposes the messages are organized into blocks of data packets. 7 In a packet-switched network the user submits his message to the network unaware of how the message will be organized for transmission. The software that controls the network must incorporate a method for extracting messages from packets when the packets reach their destinations. Security of Communications Secure network communications require that the routing algorithm be correct, that nodes cannot be fooled into sending messages to the wrong recipient, and that the physical communications links are secure from unauthorized interceptions. Since a network by its nature involves access to many computer systems, it affords potential saboteurs a chance to access many different computers. Both the ARPA network and the AT&T telephone network have been fooled on many occasions into permitting unauthorized access to the network and, in the case of the ARPA network, to computers on the network. The managers of both networks continually try to improve their protection against such access, but no workable foolproof protection techniques have been found. 8 As noted by Lawrence Castro, Chief of the Office of Research and Development at the National Computer Security Center, (continued from previous page) L?7EE Trans. on Corrmunications, vol. COM-26, No 12, December The reader should keep in mind that the ARPANET was designed as an experimental network, and not as a high reliability network intended for commercial use. Depending on the situation, several messages maybe combined into one packet, or one message maybe split across several packets. In either case, the benefit of packet switching is that network resources may be shared, leading to more efficient routing of messages and more efficient use of the network, The disadvantage is that the job of the routing algorithm may be complicated, and routing may become more difficult to debug. Access to a network is frequently separated from access to the computers using the network. Entrance to the ARPA network is through computers dedicated to that job, known as ter- Current computer networking technology has concentrated on providing services in a benign environment, and the security threats to these networks have been largely ignored. While literature abounds with examples of hackers wreaking havoc through access to public networks and the computers connected to them, hackers have exploited only a fraction of the vulnerabilities that exist. Techniques need to be developed that will prevent both passive exploitation (eavesdropping) and active exploitation (alteration of messages or message routing.) 9 Gaining unauthorized access to a BMD communications network would at least require communications technology as sophisticated as that used in the design and implementation of the network. Furthermore, an enemy would have to penetrate the security of the data links, which would likely be encrypted. Since network communications would be used for coordination among battle managers, and would probably involve transmission of target and health data," 10 the worst result of compromise of the network would be that the enemy could control the system for his own uses. Disruption of communications could result in disuse or misdirection of weapons and sensors, causing the BMD system to fail completely in its mission. To achieve such disruption, it would not be necessary for a saboteur to gain control of a battle management computer, but only to feed it false data. Less subtle ways to achieve the same means might be to destroy sufficient rninal access computers (TACS). Until recently, such access was available to anyone who had the telephone number of a TAC. Several s~called hackers have made use of TAC facilities to gain entrance to Department of Defense computer systems connected to the ARPA network, and they have been successfully prosecuted for doing so, Partly as a result of such unauthorized use of TACS, password protection has been added to TAC access procedures. The telephone companies wage constant war against people who attempt to use their long distance networks without paying. Lawrence Castro, The National Computer Security Center s R&D Program, Journal of Electronic Defense, vol. 10, No. 1, January The health of a resource, such as a sensor satellite or weapon satellite, is how well the resource is able to perform its mission and what reserves are available to it. Example measures of satellite health are battery power and efficiency of solar cells. For a BMD satellite, such as a carrier vehicle for SBIS, additional data specific to the function of the satellite, such as number of SBIS remaining, would be included.

210 203 Since most of these issues are resolved in software, the solutions chosen have a strong effect on the complexity of the software and the reliability of the battle management system as a whole. The more critical of these issues are discussed in the following sections. In almost every case, the trade-off is that adding sophistication to the algorithm(s) used to solve the problem results in software that is more complicated and more difficult to debug. Network Topology The arrangement of interconnections among nodes is known as the network topology. In attempts to improve the efficiency and reliability of networks, numerous topologies have been tried. As an example, until recently the AT&T long distance telephone switching system used a hierarchical topology to establish a circuit to be used for a long distance call. 5 Nodes were organized into levels. Messages requesting the circuit were sent from a lower level to a higher level, then across the higher level and back down to a lower level. If all messages must pass through one or two nodes, then under heavy loads those nodes may form bottlenecks that decrease network performance. If the nodes break down under the traffic load, the network cannot not function at all. As a result, most networks employ algorithms that decide what route each message will take through the network. The route may vary according to the prevailing load conditions and the health of the nodes in the network. Routing in Networks In geographically distributed networks with many nodes, the routing algorithm is a sophisticated computer program. Frequently, network performance degrades as a result of incorrect assumptions or errors in the design and implementation of the routing algorithm. Find- 5 T0 help alleviate bottlenecks in the system, AT&T is now moving toward anon-hierarchical system where nodes can communicate directly with each other rather than going through a hierarchy. Note that decisions to change the structure of the long distance system are made as the result of observing its behavior over extensive periods of use by millions of subscribers. ing and correcting the reason for degraded performance requires knowledge of the network status, including traffic loads at nodes and health of nodes. Since traffic load in particular varies second-by-second, debugging network routing software is a difficult and timeconsuming job. One can only have confidence in relatively bugfree operation by permitting the network to function under operational conditions long enough to observe its performance under varying loads. Stress situations, e.g. especially heavy traffic conditions, tend to cause problems. In operation such conditions are relatively infrequent; they are also hard to reproduce for debugging purposes. Nevertheless, for a dedicated network such as a BMD communications system, it may be easier to simulate heavily loaded conditions than for a commercial network. Either software failures, such as an error in a routing algorithm, or hardware failures may cause catastrophic network failure. In December, 1986, the east coast portion of the ARPA network was disconnected from the rest of the network because a transmission cable was accidentally cut. Although the ARPA network had evolved over more than 15 years, an opportunity for a single-point catastrophic failure remained in the design. Sometimes the interaction of a hardware failure and the characteristics of a particular routing algorithm CM cause failure. In 1971 normal operations of the ARPA network came to a halt because a single node in the network transmitted faulty routing information to other nodes. Transmission of the faulty data was the result of a computer memory failure in the bad node. Based on the erroneous data, the routing algorithm used by all nodes caused all messages to be routed through the faulty node. The routing algorithm was later revised to prevent the situation from recurring, i.e., the software was rewritten to compensate for certain kinds of hardware failures. 6 For a more complete description of this problem, see J. McQuillan, G. Falk, and I. Richer, A Review of the Development and Performance of the ARPANET Routing Algorithm, (continued on next page)

211 205 communications nodes that routing algorithms become overstressed and fail, or to destroy sufficient nodes that battle managers can no longer communicate with each other. The former attack requires that the enemy have some knowledge of the routing algorithms used; the latter may require considerable expenditure of physical resources such as antisatellite missiles. Even passive observation of a BMD communications network could reveal enough about the battle management and communications algorithms used by the network to permit an enemy to devise means of circumventing those algorithms and thereby rendering the defense partially or totally ineffective. To prevent an enemy gaining such knowledge by observation of communications, encryption of communication links and techniques for disguising potentially revealing changes in message traffic would have to be incorporated a network design. Although encryption and other technology could make passive exploitation quite difficult, a saboteur could perhaps gain access to the communications software and hardware. Analysis of the sabotage questions, however, beyond the scope of this study. Achieving secure, reliable, adequate communications requires the conjunction of at least two technologies. The technology for physical communications, such as laser communications, needs to provide a medium that is difficult to interceptor j am and that can meet the required transmission bandwidth. The network technology must provide adequate, secure service for routing messages to their destinations. SIMULATIONS AND THE NATIONAL TEST BED Preceding sections have discussed the role of computers during battle. Computing technology would also play a key role in preparation for battle and in maintaining battlereadiness. Computer simulations (box 8-A) would be needed: to anticipate threats against the system, to model different ways in which the system might work, to provide a realistic environment in which system components may be tested during their development, and to test the functioning of the system as a whole, both before and after deployment. Simulations and Systems Development Simulators are useful during all stages of the development of complicated systems. During the early stages of the development of a system, simulators may predict the behavior of different system designs. An example is simulators that predict stresses on parts of a bridge for different bridge designs. During the middle stages of development, simulators may test individual components of a system by simulating those parts not yet built or not yet connected together. An example is a simulator that reproduces the behavior of the different parts of an aircraft before the aircraft s systems are integrated. A radar simulator can feed data to the radar data processor before the radar itself has been finished. During testing, a simulator can be used to reproduce the environment in which the system will operate. Avionics computers and their software are tested before installation by connecting them to an environmental simulator that reproduces the flight behavior of the aircraft s systems to which the computers will be connected when installed in the airplane. After deployment, simulators test the readiness of systems by mimicking the environment-including stress conditions

212 206 Box 8-A. Simulations A simulation is a system that mimics the behavior of another system. The difference between the simulation and the system being mimicked (called the target), is that the simulation does not accurately reproduce all of the behavior of the target. Behavior not accurately reproduced is either unimportant to the users of the simulation, unknown to the builders of the simulation, or too expensive to reproduce. Many simulators operate by solving a set of mathematical equations that predict the behavior of the target system under the desired conditions. This process is known as modelling the behavior of the target, and such a simulator is often called a model. Others may do no more than supply a previously determined sequence of values on demand or at fixed time intervals. Airplane flight simulators are good examples of simulators. Flight simulators used for pilot training reproduce flight conditions well enough to help train pilots how to fly, but not to grant them licenses. No one would trust a pilot all of whose flight time was logged on a simulator. Flight simulators are just not sufficiently accurate reproductions of flight conditions to ensure that the pilot knows what it feels like to fly a real plane. However, a pilot who already has a license may use a simulator to qualify for another aircraft in the same class as his license, e.g., a pilot qualified for a DC-10 could qualify to fly a Boeing 747 based only on simulated flights. Constructing an accurate simulation requires that the target behavior be well understood and that there be some method for comparing the behavior of the simulator with the behavior of the target. In cases where the physical target behavior is unavailable for comparison, simulator behavior may be compared to other simulators modeled on the same target, or to predictions made by mathematical models of the target. (In cases where the simulator itself is a model, a different model may be used for comparison. If a different model is unknown another simulator already known to be reliable, or hand calculations, maybe used.) A simulator that models the trajectory of a missile in flight can be checked against actual missiles and the equations of motion that are known to govern such trajectories. A simulator that models the behavior of the Sun can only be compared to observed solar behavior, and may be quite inaccurate when used to predict behavior under previously unknown conditions. for which it is critical that the system operate correctly. Such simulators are often build into the system and contain means of monitoring its behavior during the simulation. The design of the SAFE- GUARD anti-ballistic missile system of the early 1970s incorporated a simulator called the system exerciser to permit simulated operation of SAFEGUARD during development and after deployment. Current Battle Simulation Technology attacking a single ship. Such simulations can now be run 30 times slower than real time, i.e., 30 seconds of processor time devoted to running the simulation corresponds to 1 second in an actual engagement. However, the demand is now to develop simulators that can model many missiles against many ships. Work at the U.S. Army s Strategic Defense Command (USASDC) Advanced Research Center (ARC) is representative of current BMD simulation technology. In late 1986, ARC researchers completed a set of mid-course BMD battle simulations. The simulations empoyed As faster, deadlier, and more expensive 6 Digital Equipment Corporation VAX 11/780 weapons, such as guided missiles, have been computers coupled by means of shared memadded to arsenals, the demand on simulation ories. Four of the computers could simulate technology to analyze their effects has in- battle managers, one simulated surveillance creased. For example, in the early 1970s, sin- sensors (all of the same type) and weapons gle engagement simulations modeled such (ground-launched homing interceptors of the events as defending against a single missile Exoatmospheric Reentry vehicle Interceptor

213 207 System), and one simulated 32 other engaged platforms. The ARC researchers ran three battle management design cases: 1.36 battle managers that communicated among each other, known as the distributed case; 2. a single centralized battle manager; and 3.36 autonomous battle managers that did not communicate with each other, known as the autonomous case. The centralized and distributed cases assumed 236 and 237 interceptors respectively, and the autonomous case assumed 660. The maximum threat simulated was 1,000 objects, which required 7 hours to run. The centralized and distributed cases took 3 and 4 hours respectively to run against a threat of 216 RVs. The simulation took 15 months to develop, and included about 150,000 lines of code, much of it in the Pascal programming language. (Code for the battle managers was replicated for some simulations; the replication is not included in the 150,000 lines.) Perhaps the largest stumbling block in running larger scale and more realistic simulations for the ARC is the lack of computing power. SDIO expects the EV88 experiment sequence, running through fiscal year 1990, to conduct larger scale simulations involving the ARC, the Airborne Optical Adjunct, prototype space-based BMD components, and the National Test Bed. This series of experiments will require considerably more computing power than is now in place at the ARC. Simulation experts agree that computing power is currently the major limitation in performing large scale simulations. However, other factors complicate the situation. Where equipment or environments are not well-understood or include many random variables, the accuracy of simulations is difficult to verify. This is the case, for example, in simulations of sea conditions surrounding missile v. ship engagements. 11 Depending on the architecture being simulated, the other platforms were either battle managers or sensors. Some military simulation experts noted to OTA staff that every time they performed simulated threat assessments without prior access to the real equipment being modeled, the behavior of the real equipment surprised them. They strongly emphasized that it was only when a simulation could be compared to an actual experiment that the verisimilitude of the simulation could be checked. 12 The implication for BMD is that actual Soviet decoys and missiles would have to be examined and observed in operation to simulate their workings accurately. Similarly, the battle environment, including nuclear effects where appropriate and enemy tactics, would have to be well understood to conduct a battle simulation properly. The National Test Bed The SDIO is sponsoring the development of a National Test Bed (NTB) a network of computers and a set of simulations to execute on those computers. A threat model is to simulate the launch of Soviet missiles and display their trajectories after launch. Another model would simulate a complete BMD battle to exercise a deployed BMD system. The NTB would be utilized in all phases of the development and deployment of a BMD system. It should permit experimentation with various system and battle management architectures, battle management strategies, and implementations of architectures. It would be the principal means of testing BMD system components and subsystems as well as the entire BMD system, thereby providing the basis for their reliability. Preliminary design work studies for the estimated $1 billion NTB were completed in December Initially, the NTB is to be a network of computers, each simulating a different aspect of a BMD engagement. The number of computers linked for any particular engagelzexwrience cit~ here is drawn from discussions with Scientists from the Naval Research Laboratory s Tactical Electronic Warfare Division about simulations of Naval warfare. 3Major James Price, SDIO S assistant NTB director, described the NTB as a $1 billion program through 1992 in an interview reported in Defense Electronics in February, 1987.

214 208 ment would vary depending on the completeness and depth of detail required. Initial capabilities would not permit simulation of a full battle involving hundreds of thousands of objects. A major use of the NTB would be to conduct experiments with different BMD technologies and strategies. The currently visualized NTB would link sensors, weapons, or battle managers to simulations that reproduce the data they would handle during a battle. The object could then be tested under varying conditions. The results of such experiments would be quite sensitive to the verisimilitude of the simulations. Accordingly, it is important that there be a way to verify the accuracy of the simulations used in NTB tests and experiments. Computers in Support of BMD System Development A BMD system to counter the Soviet ballistic missile threat might be the most complicated system ever built. It would involve the use of many different technologies, the automated interplay of thousands of different computers, sensors, and weapons, and the development of more software than has been used in any single previous project. Accordingly, managing the development of such a system would require considerable computer support to track progress, to identify problems, and to maintain the status of components under development, in test, and deployed. Computers would also be used to design, generate, and test system hardware and software. Engineers and managers are likely to be geographically dispersed and would need to transfer information from one computer to another. The interaction among people would only be effective if there were a means for effective interaction among the computer systems that they use. Previous sections of this chapter have concentrated on the role of computers in the operation and testing of a BMD system. But it is clear that effective computing technology would be needed not just in a strategic battle, but long before system deployment and throughout the lifetime of the system. Computing Technology Trade-offs Chapter 7 and the preceding sections have portrayed some of the trade-offs involved in using computers for ballistic missile defense. The following list summarizes those trade-offs. Processing power required v. volume of data communications among battle managers. Sharing information among battle managers relieves them of some of the tasks that they might otherwise have to perform, and decreases the processing load on each of the battle managers, but increases the data communications rate requirements and also requires that communications be secure and reliable. Performance v. volume of data communications. Sharing data among battle managers allows the system to operate more efficiently, but, as in the previous tradeoff, greater dependence on communications requires greater communications capacity, reliability, and security. Performance v. degree of automation. Permitting human intervention during a battle degrades performance under some conditions, but may permit recovery from failures caused by the inability of an automated system to recover from unanticipated and undesired events. Processing power required v. battle management organization. A distributed organization would require less processing power from each computer but more communications than centralized battle management, which requires placing a considerable concentration of processing power in one computer system. Software complexity v. battle management organization. A hierarchical battle management architecture simplifies the software design but may leave the system less survivable because of the possibility of command layers being disabled. A decentralized battle management structure would increase the complexity of the communications software and might require more weapon resources, but might result in a more survivable system. Software expense and reliability v. hardware customization. Customizing hard-

215 209 Photo credit: Electronic Systems Division, US Air Force Systems Command Computer simulations are to play a key role in the development and testing of SDI systems. The photographs here are of video displays screens generated by a ballistic missile defense battle simulation program developed at the MITRE Corporation. The circles in the scene above depict areas of coverage for a system of space-based interceptors. The scene below indicates the tracks of ICBM boosters a few minutes after launch.